Allow switch to expensive territories by paying the difference

[Soxasora]

Describe the problem you're trying to solve

You can only change the territory of a post if the new territory has equal or less post cost.

Describe the solution you'd like

Allow switching to an expensive territory by paying the difference between your old territory and your new territory.

If for example you paid 100 sats to post on ~Design but you actually wanted to post on ~Photography where a post costs 250 sats, pay only 150 sats.

Describe alternatives you've considered

Deleting a post and re-creating it in the correct territory, paying full price.

Additional context

#890604