I think it depends on where the Elan system is right now. Are you going to have to work harder getting new equipment to work with the old system? Or is everything new enough to not make a difference?

We had a Phast project once where the clients sold the home. New customer wanted all new gear in the library (main tv watching area). Shades lights AV Surround etc. Rather than trying to rig the 10 year old Phast system we just sold a new independent Crestron system for the Library. Any repairs or upgrades became part of the new Crestron system and the Phast system got phased out.

If the current system is still current i would just add the new zones and be done with it. Having guests download an app IMO sounds like a bunch of weekend drunk texts/calls for support to me.