Switching plans during trial is certainly possible with Chargebee. The new plan configurations will override the existing ones. If the new plan does not have trial days, credit card details need to be present on the subscription and the customer will be charged immediately.
If you are changing to a plan that has more number of trial days than your current plan, the subscription will automatically update it to the new plan and deduct the number of days already used.
Initial Plan Trial: 10 Days
Trial days used: 5 Days
New Plan: 30 Days
Trial Ends: 25 Days after the new plan is started.
If the new plan has a lesser number of trial days than the current plan, the customer will be invoiced immediately.
Initial Plan Trial: 14 Days
Trial days used: 10
New Plan: 5 Days
Trial Ends: Immediately
The new plan will update the subscription to be active and attempt to charge the customer because they have already used 10 days of trial and the new plan has only 5 days in it.