You can upgrade from Premiumn Economy to Business Class on BA , however it will noit be cheap

You will need to book a BA fare in "W" class , which is the fully flexible fare class

It will likely be cheapre to book a discounted business class ticket than buy a W class ticket and then use 25,000 miles to upgrade

if you are going to book a W class fare anyway, then no issue

For example - the business class fare on those dates is $4172 inc taxes ( I class fare)
The W class Premium Economy fare is $6177 inc taxes

You will need to book with AA or ensure that there is an AA flight number in the booking in order to upgrade using AA miles

If there is award availability for business class, then should be no usue in upgradung - if there is a problem, it will be a fully refundable fare anyway