Back filling “left over” seats?

We sent ours in February. Our wedding is in October. I've had zero headaches doing it this way. We wanted our guests to be able to request off work, book flights and hotels and save up to come. I don't get why so many people are against sending invites early. I'm a planner and there's no way I'd be able to make it to a wedding with only a few weeks notice.

I think it depends on how your friends/family would react knowing they were on a "B List". For example, this wouldn't bother me at all but my cousin would have her feelings so hurt she would cry. It's about knowing your crowd. I have some co workers that would like to celebrate with us but I haven't been at this job for very long and they don't know FH well. If I end up having enough no's come back I may extend the invite to them.

Personally, I only think a B list is rude if it's glaringly obvious.

I would not be upset if you were up front with me and said "Hey, I wanted to invite you initially, but due to our venue seating I had to wait until I got some RSVP's back from our family members. A couple seats opened up, would you be interested in coming to our wedding? Here's an invitation with an RSVP card, please let me know by the RSVP date!"

At the end of the day, it depends on how well you know your crowd. Do you think they'd be offended? The problem is too is that if you start inviting people to fill those empty seats, you may run into drama like "a seat opened up and you invited her and not me??", etc.
Good luck in whatever you decide to do and congratulations!

Don't do it, it's not really polite and people will know if they were b-listed if its close to the wedding.

How late are you sending out the invites to your "b-list"? I don't necessarily think it's a bad thing, as long as they don't know they weren't on your original list. I'm pretty sure they would know, unless your 1st guest sent back their RSVP almost immediately and you sent the new invite out the same day.

Aside from the whole B-listing thing and what you should have done... what will you do when people that received your invite say or rsvp that they can't make it for whatever reason and then plans change in August or September and now they can make it?

Relying on someone saying that they are not able to make so far in advance AND inviting others based on that could be setting yourself up for exceeding your maximum amount budgeted for or, much worse, maximum allowed.