Basically, yes.
You can only be affected by one polymorph spell at a time. If a new polymorph spell is cast on you (or you activate a polymorph effect, such as wild shape), you can decide whether or not to allow it to affect you, taking the place of the old spell. In addition, other spells that change your size have no effect on you while you are under the effects of a polymorph spell.
So what you are doing is using a new activation and replacing the old activation. You would lose whatever time was remaining on the old activation though.
You do not need to wait until the previous activation expires. You can activate it again and replace the previous activation.
Example:
I activate Wild Shape (first use) to turn into a Leopard for 5 hours (out of 6). At hour 5 I decide I want to become an Eagle.
I activate Wild Shape again (second use) to turn into an Eagle without going back to my normal form. I now have 6 hours of being an Eagle.
The remaining hour of Leopard (from the first Wild Shape) is lost because the second use of Wild Shape replaced it.
Note: I could have turned into a Leopard again, I used Eagle to avoid confusion.