I thought once the season started you can't add anyone to your taxi squad, but you can promote current ones to full time roster anytime? I see atleast a couple franchises demoted to TS after the season started.

FW dealt with the improper demotions. However, I think we also have a limit on when you can activate them. Any player still on the TS at the start of the first game in week 13 must remain on the TS for the rest of the season, unless traded.