Changing BPM does change the grid but the thing is, with default setting the MIDI changes (scales) also so it's not useful in your case.
But you can disable MIDI items following BPM via rightclick on MIDI item -> Item properties -> Properties... (the button at the bottom of the Media Item Properties dialog) -> tick "Ignore project tempo use..."
Also File -> Project Settings -> Timebase for items/envelopes/markers must be set to "Time" for it to work.
Here's a rundown of the procedure
and here my favorite method