I think this thread got out of hand, with people trying to explain it way sorts
When someone has been issued a “1-year extension of stay” based on retirement, started out from a Non-Imm-O visa, he ends up holding a “one year stay permit” which is valid until the date stamped in the “extension of stay valid until (date)” stamp.
In case the holder of this stay permit wishes to exit Thailand and come back later during the validity of the “extension of stay permitted until” date stamp, he needs a single re-entry permit. This permit costs 1000.- THB on Immigration. In case the holder of the year stay permit wants to exit and re-enter a multitude of times, the best advice for him would be to buy a “multi re-entry permit” on Immigration which will cost 3800.- Baht.
Both of these re-entry permits will exactly be valid until the same date as the validity of the extension that is stamped in the passport.
In case, as described by the threadstarter, his stay abroad will extend past the validity of the extension, all re-entry permits become void, and also the stay permit will expire while he is abroad.
In this case, if he wishes to return to Thailand, he needs to start the whole process applying for the 90-days Non-Imm-O visa, again
The advice of
@Marty ********
is valuable in sorts of, that because you already got the proof of funds for an extension of stay permit based on retirement sitting ready in your Thai bank account, you could also enter Thailand with a Tourist visa and ask Immigration to "convert" this TR visa to a 90-days Non-Imm-O based on retirement, and from there back to the 1-year extension of stay permit