I have used Megascans in other tools before switching to Unreal Unlimited. Do I need to license those assets under a Quixel subscription if I'm using them for commercial or non-commercial projects?

Yes. Your right to use and share Megascans Content depends on what, if any, Megascans plan your account was enrolled in at the time you accessed the Licensed Content and whether you qualified for your Megascans plan at that time.

Once you switch to Unreal Unlimited, any new assets that you download under that plan are treated UE-Only Content as described in the Content License Agreement. If you want to keep your old subscription intact, you may create a secondary account and use that for the Unreal Unlimited plan.