As far as i'm aware there's no specific thread but you can search generally for "Convert 3790 US/EU to SG/MY", maybe there's something from years back. I seriously don't recommend that conversion unless you have a deep understanding of the various files specific for both devices. Otherwise, the safest way is to do a hybrid-conversion after removing all region-specific files then painstakingly gradually add files back individually and power it on each time to see if it soft-bricks.

As for a 3490 conversion, with the correct Voice Command related files (ASR2 only) and a full NTU EU map the voice command function should work. The map (NT or NTU) you have on-board can be checked via "My Maps" in settings; or connecting to a computer and looking at the content of GarminDevice.xml file found in the Garmin folder.

Unfortunately Garmin device regions cannot be accessed by a computer in that way, other than of course the file system region (either 48 or 83) which has format of FAT32 and is therefore visible, writable etc. and can additionally be formatted and checked by ALL the usual tools just like any other USB flash drive can be treated, including using CHKDISK etc.. All other populated regions are effectively invisible to a computer.

The only way to 'see' the content of region 41 is to firstly decrypt a dumped BIN file copy of it (unlike other regions, nonvol is encrypted). We then can look at it's data in a hex editor. There's little practical value in doing that usually anyway - if you did know what data to remove, replace or add, then after making changes you re-encrypt it's data and manage to flash it back it to a healthy device the nonvol just gets updated/rewritten again on the next boot-cycle. I don't think your faulty device can even access rgn41 regardless, because it can't be erased using "ergn,41" text command. - that's essentially why it can't be restored as US/EU 3790.