If everyone can agree to trust one person not to run off with their land then a group would work fine.
You (for simplicity's sake, it can be anyone) create a group and make sure that abilities are set so that noone but the owner (you) can sell the land but that people can deed land to the group.
If A and B want to swap land, you'd invite both into the group and they would deed their respective parcels to the group (set the land to group, check "Owner makes contribution with deed"
which puts the land and the tier into the group.
You then set B's parcel for sale to A at L$0 and A's parcel for sale to B at L$0. A and B take their tier contribution back out of the group (gets a warning, but nothing will happen in the time it takes to complete the swap) and buy the other's parcel and noone goes up in tier.
---
There are other scenarios as well, but you'd always end up deeding the land to one group or another and taking the tier contribution back out to purchase the other's parcel.
---
Few (non)obvious caveats:
* make sure auto-return is turned off on all parcels before doing anything
* if people have more than one parcel on the sim and they're going to swap then they need to acquire another parcel *before* they sell one of theirs. If they sell theirs first it could potentially put them short on prims and random things will get auto-returned
* landbots are still about so double and triple check (or stress to everyone) that parcels need to be put for sale at L$0 (assuming it's a voluntary free swap) which forces you to name a specific resident to sell the land to
