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Just accept lowres old drawings as part of the medium.
First of all, 10 years from now you will be way more capable of drawing something good to fit your high-res need.
How many of your old pieces do you think will need to be "restored" for some purpose?
Second, as I said when opening this post, it's part of the medium. The same way we look at VHS recordings or yellowed paper, lowres pics that have been upscaled will be just that, a product of their time.
If you really want a numeric answer, think about what is the "real life size" of the picture and from there you should calculate its DPI.
72dpi for digital display, 300 for printing.
So if your picture is supposed to be viewed as 10x10inches, you do it at 720x720 for digital storage, 3000x3000 for future printing.
That said, I've upscaled many a 72dpi A4-sized drawings into 300dpi to put them on my wall and they look good enough to me, 300dpi is probably companies erring on the safe side (and then some ask for 600dpi).
Finally consider all the JPGs and MP3s you have saved and whether it'd be worth to have them all in PNG and FLAC. If that sounds non-negotiable to you, then sure, shoot for your future-proof 600dpi TIFF copies, if it's a scan don't forget to store 3 different exposures, normal, under and over-exposed for maximum range.
