FaceRig will be removed altogether, with all DLCs and upgrades, so that means that you won't be able to switch to Pro once you pass that commercial threshold. I'd recommend switching to Animaze in that scenario; its Pro subscription tier is necessary once the user has passed $500 a month in revenue. However, you are free to choose to which software you switch, so it's up to you in the end.

As a side note, FaceRig's been on a support sunset for a year now, it being a seven year old software. I think it should be purchased for nostalgia value rather than commercial reasons, since legally, we can't sell any other derivate product of FaceRig - such as the Pro license anymore (once FaceRig is removed from Steam's store, that is).