I have retirement extension and it valid until expiry date of passport in 2022. If I leave country and get anew passport but the old one still has the valid visa extension , and the old passport has not expired .I have been told by a visa agent that the visa is still valid and can still enter Thailand with  new passport and the valid visa in old passport ( not expired). And can get it transferred when back in Thailand.  Hope you understand and the visa experts mod will advise me. 

You can enter the country by showing both passports to use your re-entry permit in the old passport. They will stamp you into the country in your new passport.

After entering the country you will need to go to immigration to have your stamps transferred.

What is the longest period of time a re entry can cover ?

3 hours ago, pixelaoffy said:

What is the longest period of time a re entry can cover ?

A e-entry permit is issued for and is valid for the length of your current extension of stay.

Without a re-entry permit you would lose your extension and be stamped in with a visa exempt entry.

33 minutes ago, ubonjoe said:

A e-entry permit is issued for and is valid for the length of your current extension of stay.

Without a re-entry permit you would lose your extension and be stamped in with a visa exempt entry.

I thought a re entry could have a shorter time period than the date of extension or visa expiry 

24 minutes ago, pixelaoffy said:

I thought a re entry could have a shorter time period than the date of extension or visa expiry 

Nope.

It is matched to your current permission to stay.