Plugging a slower network into a faster one shouldn't inherently cause any issues.
But just a heads up for anyone considering this upgrade. Most firewalls will struggle to do 150Mbs, even if they have gigabit ports. If you don't end up getting full speeds it may well be because your own firewall can't handle it.
And on a related note; Shaw has 2 modems available for this service, Cisco & Hiltron. We've experienced serious issues with the Cisco modem (almost certainly because it's been miss-configured by Shaw). Even in bridging mode the modem is dropping UDP packet fragments. If you experience any weird "stalls", it may be due to this. Specifically, the Cisco modems do not work with SIP UDP and Shaw only uses the Hiltron modems modems with their own VOIP service.