Nothing *really* changes in the city of Athkatla once you actually return with Imoen (though it takes two entire chapters to get back with her).

Three new areas open up, but there is not much reason to go to any of them.

Ribald Barterman, of the Adventurer's Mart in Waukeen's Promenade, will start showing off his "special stock" - you have to pay 50 gp to see it, but can buy some much higher quality items there. However, by this time, the chances are - apart from the high-level spell scrolls - that you've got other equipment to match it.

There isn't actually a time limit on going to rescue Imoen, but there may be a time limit on other sidequests - particularly those of certain party members. Nalia, in particular, may leave if you do not liberate De'Arnise Keep fairly soon: in any case that particular one is one of the easier of the "major" sidequests.

It will be strongly hinted, during Chapter 6 (when you arrive back with Imoen - if you choose to accept her into your party at all, you might go right through and not bother with her once you've actually "rescued" her from the prison, she will make her own way back separately and stay at the Copper Coronet) that time is very much of the essence. However, this is not in fact the case: you will be able to go around and complete any Athkatla-based quests that you missed out previously.

One thing, however, will change: Some areas have "random encounters" that are scaled to your party, and you may encounter the tougher versions of these if you do the sidequest containing them later rather than earlier. For instance, in De'Arnise Keep (which you can leave till later, nothing will change except Nalia won't want to stay in your party for long if you don't jump to it), some random encounters are merely Trolls or plain sword-wielding Yuan-Ti: if you come back later the former may be Giant or Spirit Trolls, and the latter may include a Greater Yuan-Ti and one or two Yuan-Ti Mages.