Well, Tvad is right as long as this is occurring inside one amp- like if you have a switch to go back and forth. But- if you compare real triodes to pentodes **wired** as triodes, you will hear a substantial difference.
Ultralinear is a way to approach triode linearity with a pentode, without loosing a lot of power. It is a compromise. So is wiring a pentode in triode mode. If you really want to hear what a *real* triode **actually** does, you will have to get a real triode amp to do that. The difference will not just be 'something different' (all other things being equal), it will be a real improvement.
Ultralinear is a way to approach triode linearity with a pentode, without loosing a lot of power. It is a compromise. So is wiring a pentode in triode mode. If you really want to hear what a *real* triode **actually** does, you will have to get a real triode amp to do that. The difference will not just be 'something different' (all other things being equal), it will be a real improvement.