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The subject tells everything. How could I change it? For example, replace grass outside Pallet town's map border with...I dunno, forest tile?
The borders for maps are blocks, not tiles, which is useful for, like your example, having tree blocks border a map.
The first byte of any map's object data is an id for its border block.
From the pokered disassembly:
PalletTownObject: ; 0x182c3 (size=58) db $a ; border tile
Hover, that should read "border block".