Hosted Page - Upgrading user from LIVE £0 subscription to 30 Day Trial Paid Plan Fails

Hosted Page - Upgrading user from LIVE £0 subscription to 30 Day Trial Paid Plan Fails

We have 2 subscriptions on our website managed by Zoho subscriptions..

One is a plan that is setup with a cost of £0 the other is a £20 per month recurring subscription.

Upgrades from the £0 to £20 plan are done via a hosted page, which worked fine until recently we added a 30 day FREE Trial period to the £20 plan.

Now when a user tries to upgrade from £0 plan to the £20 plan(with 30 day FREE Trial)  we get an error stating "Downgrades aren't permitted via hosted pages"

Could this be because the plan is trying to move from a "Live" plan to a "Trial" plan?   Any help appreciated!

Thanks