26" to 27.5" conversion

I have a 2010 Specialised Rockhopper with 26" wheels and coil front suspension. I plan on getting a new hardtail with 29".

As the Rockhopper would become obsolete, I don't want to sell it so am looking to make into a simple commuter type bike.

I want to get some rigid forks and slimmer slicker tyres however would i be able get fit 27.5" wheels with maybe 1.5" slick tyres onto the frame ? (26" is currently running 2.1")