We have a TON of good F40T12 bulbs in new or like new condition, mostly Sylvania F40T12s 4100 and 3000K that are leftovers from when we converted several church buildings over to T8.  Unfortunately (or maybe fortunately) we have lost since exhausted our supply of T8 lamps and will not be able to order anymore until January or February due to budgeting issues.  In the mean time, I've gone ahead and started putting back in these T12 lamps into the T8 fixtures where the T8s have burned out, and the only thing I have noticed is that the light output of the T12s may not be as bright as that of the T8s, but otherwise they seem to work just fine.  These are 34 and 40 Watt T12s in place of the 32 Watt T8s.  What kind of damage am I risking by running T12s in T8 fixtures?  They seem to do just fine both with the F40s and with the "U" bulbs. 

You'll likely kill off your T12s faster because you're underdriving them without cathode heating.

You'll get a strange end blackening like you just developed rings inside.

well good deal!  I am starting to notice the strange end blackening a bit on the ones I've had in for over a few months now.  But hey at least I'll be able to thin down our collection of 100 or so T12s that would otherwise just go to waste by being recycled with full life in them.  I'm keeping about 4 boxes of the new T12s which I am going to store somewhere and sell for lots of $$$ when I am old and grey LOL 

I've seen plenty of occasions where T12 was replacing T8 and didn't notice any adverse effects, depending on ballast. I guess if they happen to be electronic 'rapid start' T8 ballast, that would be the best for the under-driven T12's. The most annoying and possibly damaging for ballast is T8 put in 40w HPF T12 rapid start.

Nope these are just standard Rapid Start F40 T12s in electronic rapid or instant start T8 ballasted fixtures.  The ones I am using the T12s in are the older of the electronic T8 ballasted fixtures that way I figure if they do end up doing any damage, no worries, a new programmed start ballast will be put in.

If the fixtures are ON for long hours with infrequent switching (e.g. 10 hours per start for a working day or even 24/7), it does not matter if the ballast is cold starting IS or some programmed start type at all. Once the lamp is started, the starting method does not play any role anymore.
What starts to matter is the driving current shape (current crest factor) and frequency. And there the high frequency sinewave drive means less stress and wear than the 120 reignitions per second with the 60Hz feed.
Obviously the cathode thermal budget balance plays it's role, so the fact the lamp is underdriven without an auxiliary heating becomes stronger factor than if the feed is high frequency or 60Hz.

With about 6 hours per start both effects are about the same, so for shorter burning hours the starting method is, what matters.
And with longer hours per start the driving current (crest factor, frequency) become more important than the starting method.

Well that's the thing.  These are in church buildings.  We have the hallway lamps which will see about... 12 hours of use on Sunday and then it's sporadic throughout the week.  Then there are the classrooms, they'll be on for maybe 4-5 hours total on Sunday and then get turned on and off every day once when checking the rooms at night for building lock-up so a quick on to see if the window is locked, then turned off about 5 seconds later. 

Programmed rapid starts are neat, but they never really caught on and they're quite expensive. If you already have electronic T8 those drop-in T8 LED works pretty good.  The 2100 lumen 4' T8 and 2' U bent are both about $25. It's a good value for the U shape since the U lamps are always quite expensive.

If you go LED, get the lamps that run directly off the mains. The ballast is just one more component to fail, and wastes power.

I would really avoid the LED tubes. They are much dimmer than the fluorescent tubes, the light quality isn't good, and the ones that run without a ballast can have an annoying 60 Hz flicker that can cause headaches in sensitive people.

Using the old T12s in the T8 fixtures will give slightly less light and the tubes will burn out faster, but the ballast should be fine.

We won't be ready to switch over to LED Tubes for a good 10-20 years at our buildings I suspect.  But that being said, I just picked me up a LED Shoplight fixture from Costco to use in my garage strictly for trial purposes.  We'll see how well it does.  Built by Fiet Electric, so I don't expect anything spectacular but rather.. subpar.