We do our BAS each quarter but we invested significant funds into the business in Oct and Nov and want to know if we can do our BAS for the 2 months now, then do Dec at the end of the month and then go back to quarterly?

Short answer - no. If you change to monthly you will be stuck with that for a minimum period of time. Check the ATO website for further details.

Might pay to check your numbers, might not be that much benefit for the longer term frustration of monthly BAS.