Hi,
I would recommend you to simply check the code of Models and Controllers first. If the 1.6 extension utilised new 1.6 classes such as modellist, modelform, controlleradmin, etc, and the code is quite alot, then you better change your mind from downgrade the extension to upgrade your template.
However, if the 1.6 extension had not utilising new classes, that would be just few lines modifications and a little renaming stuff.
In my opinion, upgrade is somewhat you have to do finally, and there are lots of people could do it, just pay for it. On the other hand, downgrade is somewhat you will finally dump it, and it is not a normal task even you pay.