Replace A T5 Fluorescent to A T8 LED
The T5 fixtures have several differences the first is they are shorter than a T8 so no you can't use the T8 lamps.

2nd would be,
The tombstones are smaller in a T5 and not compatible with T8 or T12.

Last the T5 fixtures and lamps I use require 54w ballasts where the T8 only require 30-40w this may be a problem for both the ballast and the new led lamp if you could make it fit. 

A new fixture is the only real path forward for T8 but T5 tombstones are inexpensive if you want to save the fixture.