Picking your seat etc for a mid-season move and knowing how much you will pay for a full season at BM isn't the issue. That can easily be sorted.The club will already know the pricing structure etc now.
The issue as I see it is realising any increased reveue from season ticket holders when moving mid-season. The club can't determine at which point in the 24-25 season BM might be ready so at the point of renewal they can't charge you for X games at Goodison and X games at Bramley Moore. Well, they could but if they get it wrong then you're into the realms of credits/ refunds which would be a nightmare.
They would therefore need to come back to season ticket holders after renewal and ask for more money when they know what the definitive move date will be; if they decided to re-locate mid-season. That won't happen in my opinion. I think existing season ticket holders would be allowed to pick seats and move across to BM without paying any extra initially. Only paying the increased season ticket price at the start of the first full season, if they decided to renew.
This is why there will be little appetite for a mid-season move because, whilst perfectly doable, its a bit of a pain logistically and everything is much simpler if done during the off season.