While we all have our moments of inglorious sartorial purchases and growing pains of learning what is classical clothing, several thoughts come to mind which might be of help.
Let me preface this by saying that when I was a young man, entering my legal career, I should have been a bit more conscious of the finer points of clothing, though the basic understanding of classical style was imprinted from a young age, and reinforced during two years at Oxford, in England. In other words, fewer articles of clothing better made is best. As a rule: go first class or as close to it as you can get, and wait until you can do it right the first time.
I hope you will not take unkindly the following comments based on a brief review of your posts, which, taken singly, are all interesting questions, but when looked at together make me wonder whether you have yet grasped the confidence of mastering the fundamentals of classical style.
First, no one I have ever heard of would alter a t-shirt, whether white and under shirts, or, presumably a colored or patterned t-shirt, even if sentimental. Part of the charm of a clothing article from the past is to not look too studied or affected. Taking in a jacket or trousers or suit due to weight loss or gain is appropriate, within reason, assuming it remains age appropriate. Altering a high school letter jacket would be vain and raise the question as to why some article would justify such a disproportionate expenditure. More importantly, it would look odd when worn, regardless of the degree of sentimentality attached to it.
Second, whether or not one would put shoe trees in sneakers falls into the same category. It is like having bespoke cowboy boots to work out on the farm all day in best clothes while working amidst the cow sh**. Thus, my reductio ad absurdum.
Third, when you ask the question on the design of a class ring for an LLM class ring from a very fine law school, while it may be more common to see in the realm of working places and perhaps even law firms in certain parts of the country, it suggests inviting scrutiny to flaunt the school and your personal accomplishment. Now there is nothing wrong with this, as it all has its time or place. But if you look at DeBrett's, or Roetzel's book, you will see that an initial ring or wedding ring is acceptable, other than the signet if you are nobility or of ecclesiastical position. While this happens at the service academies in the USA, as well as the Citadel and Texas A&M, that is a peculiar military connection, which I generally feel is slightly more acceptable in that military setting.
If I may continue with this point, having earned 5 degrees, including an MA at Oxford, a JD at Georgetown, and an LLM at George Washington, as well as being a retired military officer, the only class ring I ever bought was one at my small rural high school, one of a class of 22. I wore it the senior year of high school, but never after.
In the legal profession, in DC, as well as other cities, I never saw a ring from a law school, certainly not an LLM. So regardless of whether you would wish this, which one cannot quarrel with, note that you would stand out amongst your peers, and perhaps call attention to it in a negative way.
With respect to the US military, I expected, on the other hand, to see class rings on fellow officers if they went to the service academy or A&M, as one friend was first in his class at West Point, and my brother an A&M grad. Thus, importantly, one would possibly stand out if they did not wear it.
As to the Oxford connection, it was not something that I ever thought about much in an overt way, other than simply wearing my college tie, and occasionally the warm wool college scarf when in brutal cold weather. The tie is all right, the boat club jacket at Eights Week or Henley. Sub fusc at degree ceremonies, and gowns at dinner at High Table, are also acceptable. A ring of the college is something I have never seen my Oxbridge friends wear.
In the British military, my friends who are retired or who have served wear the regimental tie, and that is sufficient to call attention to their miltary status and service. Several are also MBEs, OBE, and CBE. When in military or civilian formal dress, as the occasion requires, they will wear their decorations and orders, although I have met and conversed with more than one internationally known figure who simply wear the black tie or dinner jacket without decorations. None wear rings, though when there is ornamentation, it is largely of regimental brass buttons of the branch of service, such as are made by Benson & Clegg in London, suitable for a blazer.
Now, as to your present situation, how to address the situation of the sentimental attire of various types.
With respect to sneakers, best to wear them while they hold up, and, when finished, either donate them to charity if life is left, or put them in the attic with a label for succeeding generations with explanations as to their importance to you.
As for the t-shirt, I would confine it to wear as long as it is baggy, then relegate it to the archival portion of your wardrobe in ages past, letting the memory think of those times, rather than adapting it to the modern era of wear. An exeption, for sports buffs, is no doubt if it is a uniform signed by Babe Ruth, known in Boston, they tell me, and under glass. Short of that, it is not worth framing and putting on a wall. Though if, on the other hand, you have your jersey for winning the Boston Marathon and wish to put it on your wall, along with associated Olympic medals, then I yield.
Now as for the commemoration of universities attended, I would say to "broadcast" this in the most universally acceptable way for lawyers, which is to gather up all our diplomas, admissions to state court bar, federal courts, The US Supreme Court, and get them a first rate frame and matting job to hang in your office along with a decent nameplate to put on your desk.
As you can see, understated is best in my book. While I have not always done it right the first time, eventually it call came around to this as the best course of action, as others advised me on what was correct, much the same as I now advise you.