EnterraHost
Plans sit on a ladder rather than standing alone. Moving up or down it is something you do yourself from the toolbox, and the only part that ever refuses is a downgrade that the service is not yet small enough for.
Where the change is made
On the service itself, in the toolbox, rather than at the account level. Each service carries its own plan, so changing one does not touch the others, and two EnterraMon services on the same account can sit on different tiers.
The upgrade dialog is reachable from the service row. It offers the tiers above the one you are on, and where a service bills monthly it also offers moving to the annual term, which is a different kind of change and is worth understanding separately.
What happens to the billing
A plan change is a change to the recurring amount, not a new order. The service keeps its history, its configuration and anything it is already monitoring, and the new price applies from the change rather than from a fresh start.
Moving to the annual term is the one that behaves more like a purchase, because you are paying for a period rather than changing a rate. If you are weighing that up, the saving is stated on the pricing where the term is offered, and it is the same two months free that applies to a new annual order.
Why a downgrade can be refused
This is the part worth knowing before you try it, because a refusal looks like a fault and is not one.
Every tier has a limit on how much it can carry. EnterraMon and EnterraSEO tiers carry a number of sites, and the limit rises as you go up the ladder. A downgrade is allowed only when the service is already within the limit of the tier you are moving to.
So an Agency plan monitoring ten sites cannot drop to Pro while all ten are still there. The service is using more than the target tier permits, and the change is refused with a message saying exactly that. You remove the sites you no longer want first, and then the downgrade goes through.
There is one case where it refuses without a count, and it is deliberate. If the system cannot read how many sites the service is using, it will not move the plan down, because an unknown count is exactly the situation where the limit could be breached without anyone noticing. That refusal tells you to contact support rather than guessing, which is the honest answer when the number is not available.
Downgrades are not available on every product
Worth stating plainly rather than letting you discover it. Plan ladders exist for EnterraMon Pro, EnterraSEO and Business Email, and the mechanics of a move are added per product as each rule is settled rather than switched on everywhere at once.
Where a product has no ladder yet, a downgrade is a conversation with support rather than a button. That is not a limitation being hidden, it is a rule that has not been decided, and a support ticket is the honest route while it is undecided.
Before you move a plan
Check what the service is actually carrying, because that number is what decides whether a downgrade is allowed. If you are moving up, there is nothing to check and nothing to lose, since a higher tier carries everything the lower one did.
If the change is part of cancelling rather than adjusting, that is a different job with its own consequences, and cancelling a service covers what happens to the data and the notice period. Moving down and cancelling are not the same act, and treating one as the other is how people lose a service they meant to keep.