I have an old 7 speed touring bike that I wasn't using, but still held a lot of sentimental value for me. A bunch of years ago I considered changing things on it, but realized that it would entail too much money, so just ended up redoing the simple stuff, new hoods, all new cables and housings, bar tape--and just left it at that. Nice to see it all made up again, but starting to get into changing down tube shifters for brifters or bar end shifters, changing out the rear end to wider, diff rear wheel, cassettes, maybe derailleur...it all just adds up to a bunch of money that could go towards buying even a fairly new used bike in good shape.
do what you want, but just be aware of what changes cost.
I actually ride my old bike on the trainer in the winter now, original old gel seat and all....nice to have it around but one day may sell it (been saying that for over ten years though....)