For instance, I have been issued the single ED visa, enter date before Oct 09, 2018. I am going to arrive Thailand on Sep 09, 2018 to start my Thai couse.
From now until by that day, may I enter Thailand by free-entry of waiver visa in ASEAN for 30 days?
TLDR : Answer Summary
The user inquired about entering Thailand under a visa waiver while holding a single ED visa that becomes valid after arriving. Comments indicate that having a valid visa will typically result in the Immigration Officer applying that visa upon entry, but the officer may have discretion in such cases.