I want to develop plugin for Milestone smart client but I choose to use Essential version of trial. May I change the trial software to other version that supports plugin ? Thanks.

Sure you can, as the installer is the same for all XProtect Professional portfolio products. It’s just a matter of changing the license. For this, I suggest you request a Pilot license by contacting your local Milestone distributor or via the online License chat or by calling the Milestone HQ sales team. Thank you.