To add to what @stephen has very promptly shared already, the top-up worked flawlessly, the funds appeared circa within an hour. In fact, even my GIA top-up reference remains the same as it was prior to ISA switch in December (not sure if it is the case for everyone), hence I did not even need to copy it - clever Monzo remembered both 
I am of a strong opinion that having both accounts available is great in long-term, but personally do not see the reason to use GIA unless the ISA limit is reached or if certain stocks are only available for GIA. Speaking of the latter, hopefully, that will make it easier to offer such stocks as Tencent, which are not allowable for ISA!
Also, one other very significant factor for me is seeing the total value of the entire Freetrade Account (GIA + ISA), even as a plain number somewhere (even better if it could show the total change for 1D/W/M/Y). For example, something like this (but with fancy Freetrade design, of course):
Look forward to seeing other people’s opinions about having both accounts accessible
