Breakpoint on the tilemap in VRAM, to see where the tilemap is coming from. Change that then, you'll propably have to adjust some code a bit, only a loop counter that determines how many tiles to load most likely.

If you want to use the new space too you might have to adjust some kind of max_char variable in the text routine too, but since you said it's only used once I guess that won't be needed in this case.