I have a fixture that came with T-12 lights. Is it safe to use T-8 bulbs instead?
You are meant to replace the ballast on old T12s, reusing the old G13 base and enclosure. There are numerous retrofit and upgrade kits available.
The official word for the USA is:
Based on [Department of Energy] analysis... most T12 lamps are too inefficient to meet the [current] standards and can no longer be manufactured for distribution in commerce after July 14, 2012. In addition, if a consumer replaces an existing T12 system with a more-efficient T8 or T5 diameter system, he or she may need to purchase a new ballast or fixture. Even with this additional cost, however, DOE has determined that increasing the energy conservation standards will save consumers money overall. http://www1.eere.energy.gov/buildings/appliance_standards/residential/pdfs/general_service_fluorescent_factsheet.pdf
Mixing the two bulbs types is chancy: it might work out great for you, but probably not. Many of the old ballasts are pretty bad anyway: the new ones in this case really are better. Old ballasts are likely magnetic and might hum. They may have PCB chemicals, and they have a reputation for getting hot and even igniting.
Note: retrofit kits make sense in commercial applications. For your one fixture, it may be cheaper to buy an entirely new one.
2017 Update: The cool kids are all using LED instead. As before the hookup options are still bewildering (single ended, far side terminated, ballast bypass, ballast compatible). As before, the same G13 base and fixture can be preserved.
As far as safety is concerned, you can exchange them. If you place T12 tubes in a fixture with a T8 ballast, you will wear out the ballast and have to replace it. If you place T8 tubes in a fixture with a T12 ballast, then the tubes will have a shorter life due to a higher current through the tube.
Apparently, at least one ballast model is rated for F32T8, F40T12 lamps, the Keystone KTEB-240-1-TP /B. Also, the Fulham Pony NPY-120-240-T8 is or was made to operate either T8 or T12 lamps. T12 lamps with a ballast for T8 lamps might cause the ballast to go bad sooner, if T12 lamps (rated at 430 mA) draw more current than what a T8 lamp is rated at (265 mA) from your ballast rated for T8 lamps. If your T* ballast is designed so as to keep lamp current close to 265 mA, you should not have that sort of worry.
Probably not without also changing the ballast. You could turn off the circuit breaker for the fixture, open it up, and read the label on the ballast to check - some can be used with either type of bulb; but most of the fixtures supplied new with T-12's at this point are bottom of the line and the ballast is T-12 only.