As the bike gets slacker, the reach also gets shorter, even when the top tube length stays the same. That's because along with the head tube angle, the seat tube angle also changes, so the bottom bracket moves forward in comparison to the end of the top tube that connects to the seat tube, and therefore, the horizontal distance from the top of the head tube to the bottom bracket decreases.

The other measurement you should concern yourself with is the stack (vertical distance from the bottom bracket to the top of the head tube). A different stack height might make the bike feel taller or shorter. This can be adjusted somewhat with spacers, different rise bar, or a different angle stem though.

As never said though, it's a different bike, and no matter what, it'll ride differently. There are a bunch of things you can do to tweak the fit, and it doesn't sound like it's so different that handling will be completely compromised getting it to fit comfortably.