Are T5's, T8's & T12's interchangeable?

[Paul Mc]

I have three fixtures in my basement greenhouse that currently use T12 bulbs. I needed another light as my tropical collection has expanded and we need more room. Unfortunately, I did not realize that I was buying a T5 fixture until I tried to put T12 bulbs in them.

After researching, I found that T5 bulbs are probably better for the higher light orchids and tropicals that I keep in the basement.

So, my question is the following: Is it possible to replace the T12 bulbs with T5's in my old lighting fixtures? I know the new light I just bought will not fit 2 T12 bulbs, so that's not possible.