Electrical – Are T-8 lights and T-12 lights safely interchangeable

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.