I haven't actually had it happen myself but theortically what should happen is you'll loose access to all the premimum only acres - you can view them listed here https://horsefable.com/construction. That'll leave you with 150 acres and 100 boarding passes. If the only acre upgrades you have are stall blocks, you should be able to house 150 horses + 100 boarding passes = 250 horses on a basic account. Beyond your 150 horses for stall blocks, all of your horses should theortically be kicked from home boarding. 

Then all those horses will be in transport. There is no way to ONLY freeze your transport tab - you have to board them first before you can freeze them in a tab. You would only be able to freeze them by freezing your entire account. Otherwise they will continue to age and be unshowable until you do so. Therotically you could play this game of boarding horses, showing them, then unboarding them, boarding more to show/freeze but it'd be kind of a pain but doable depending on the amount of horses you got. 

Do you have 3 accounts yet? One of my suggestions to you would be to create/use your two extra accounts to house your extra horses at home boarding. Granted you have to buy some of those acres and stuff back with bits but it might be a bit easier to juggle then juggling all of them in your transport, depending on how many horses you have. 

If you want to do this method, I'd suggest starting the process before your upgrade runs out because you should have access to mass actions so then you'll be able to mass transfer horses to spare accounts. 

Everything is, mostly, barn for upgrades. Only the water tower and feed tank to feed/water 500 horses take up 2 acres per, then racetrack, outdoor course, then indoor/outdoor arenas take up 5 acres apiece. I probably could demolish them for time being and leaving me with only the feed/water options and rest barns.

Unfortunately, I only got started on one alt account and I keep forgetting it exists overall. I have shuffled horses to that one alt for side projects I didn't want to fully give up on, though I already retired some of those and shuffled more over to my alt. I believe that one is at max on acres and I haven't worked on it enough to buy more land/stalls.

Going off your 150 stalls + 100 for boarding passes=250, I have 311 horses and minusing that out leaves me with 61 horses extra. I'll have to look at another alt and try hustling that up first and take some over to second alt and then figure out first alt.

Thank you for the advice.

Okay, so my account was finally downgraded and this was the message that popped up: Oh no! It looks like your upgrade expired! Your estate is on hold until you can upgrade again.

I was going with your line of thinking and that my horses would be kicked to transport but it seems none of them have been kicked. I had already managed to shuffle some to my alt and retired some horses to be under the 250 threshold and actually put some horses up at others' boarding centers. I do plan on going in and shuffling everything around but will stuff still be on hold indefinitely until I upgrade even after I shuffle stuff around?