Voob, sounds like your significant other is right, and let me be among the first to thank you for writing Dear Abyss with your question. While it is possible in some cases to change tank usage specifications, it's probably easier to insert a football into the bowels of an irate Bengal tiger, using a hot poker. Noble intentions, I'm sure; but, simply not a good idea...
Sounds like the older tanks to which you refer are steel 72's, which are a little smaller in size than today's common aluminum 80's. First, the threading will not allow an standard healthcare O2 compatible valve to be inserted into them, and besides that, before they could even be used for Nitrox blends, the current valve itself must be completely overhauled, all O2 sensitive parts, and lubricants removed and replaced with Viton and other technical stuff, and the tank tumbled, washed in solution, and dried with nitrogen, in order to prevent combustion and explosion, the most likely outcome of your experiment. On top of this, you'd have to have them inspected, both hydrostatically and visually, resulting in your eventually spending more than the old tanks are worth, or more than they cost you, which should have been about five dollars or so, since that's what they're worth. While I do not mean to belittle your diving experience, your referring to "using them for O2 tanks" has me a little concerned, as no person within the vast confines of this planetary system who knows anything about sport diving would refer to scuba cylinders as "O2 tanks."
With this said, I concur with many of the prior posts which advise you to seek certification, whereby you'll gain the knowledge necessary for you to know beyond doubt that what you are proposing is extremely dangerous. Oh, and about the nitrous tanks....don't even think about it. Too involved to explain here, but suffice to say that you'd be safer while shooting fireworks while submerged in a vat of gasoline.
Best wishes on your continued survival, and kudos to your significant other. I'd listen to her.