You should never restrict an ID field to a small set of values like A or B, it makes no sense. I’d also argue that the numbers themselves being meaningful like a NSEW designator is a bad idea, for much the same reason. What if you have a mismatch of numbers of NS and EW flights, and you exhaust the EW designators but not the NS ones. If this was to use the existing flight number space resulting in a reduction to 3 digit flight numbers it’d make the problem worse, if you’re adding a digit why other than 5 digit flight numbers?

I’ve worked on systems that have suffered from a lack of suitably sized fields, management wanting 5 character order numbers so as easy to quote to customers, insisting on 3 digit batch numbers, that were exhausted in 18 months. Then watching as ‘numbers’ like AZT7I being quoted to customers after desperate changes to alphanumerics. Either you make the 4 digit numbers work somehow, or you add digits, which I suspect would be a hugely complex issue.