Recently I've changed 20,000 euro into Baht, and put it for tax reasons -temporarily- in my wife' s Thai bankaccount. Will it be OK to get this money transferred to MY Thai bankaccount in, say, 2 or 3 years to serve as my 800,000,- baht when applying for a retirement visa? In other words: do I need to prove the money came from overseas to begin with?

For a conversion to non immigrant visa proof of funds coming from abroad with a letter from the bank is required.

For an extension of stay it is not needed.

You may well be asked how 800,000 "suddenly" appeared in an account.

It would be normal o be able to demonstrate, clearly, a source of the cash.

Not intending to hijack your thread.

I was thinking of asking a similar question, as each time I am in Thailand, I change, and deposit any remaining Traveller's Cheques that I have, into a Thai bank account to build up funds for a future application for a retirement visa.

Better to take them home and send money from abroad?

Or get a single entry non-o visa visa so you don't need to do a conversion to a non immigrant visa entry. Then just apply for an extension of stay.

You may well be asked how 800,000 "suddenly" appeared in an account.

It would be normal o be able to demonstrate, clearly, a source of the cash.

Yes, my bank manager's letter for Immigrations indicates the total in the account and also all foreign sourced deposits in the past three months. It may not be a stated requirement for retirement extensions, but it is noted and, as Sceptic says, Immigrations may want to know where the money came from.