If you were building a light rail system from scratch you'd be massively unlikely to use high floor unless it had to co-exist with any existing high floor services or there was high floor infrastructure you couldn't / was impractical to change (e.g. underground stations)
High floor is undesirable because it's "non-standard" and yes, you can build high platforms on streets, but they're a pain - see Manchester city centre for numerous examples of why!
Doors at different heights is a an awful idea, not just for the awkwardness of boarding in different places, but also alighting. The interior would be at different heights, so you'd have to walk up/down steps inside to exit which would be a nightmare at busy times, and impossible for wheelchair or pram users...
San Francisco's light rail "solves" the problem with movable interior steps, but it's a rubbish solution - high level platform stations are fully accessible, some are hybrid on-street stops where one door of the train will be level, the others steps, while other stops have no infrastructure at all and are completely inaccessible to those who can't use steps. Here's a video of the changeover:
I think the sensible influencer of the choice should be whether or not any platforms will be shared by light and heavy rail either now, or in the future - is it sensible to prevent it from happening in the future? If not, you'd make it low floor and you wouldn't lower existing heavy rail platforms, you'd raise the trackbed which should be easier and cheaper.
It can be done, and is in places, but TBH I'd favour the "nuclear option" of flatten the lot and start again, one line at a time.
Only one of Metrolink's extensions involved closing an existing line and "starting again" - Oldham + Rochdale over the old Oldham Loop line. Rochdale itself still had service to Manchester, which wouldn't be the case for any of the valley lines, and being valleys the journey options between them are limited. The reason the new bits of Metrolink look new is they are new!
It did give a period of pain for Metrolink users, but it resulted in a proper European quality light rail service rather than a classic British cheapo bodge job. (The new bits of Metrolink genuinely have the feel of a German Stadtbahn type U-Bahn - even things like proper platform canopies rather than cheapo bus shelters)
No they don't, they have bus shelters everywhere other than on island platforms (mostly at terminuses) which have centrally supported canopies, just like every part of Metrolink including the oldest bits. They're functional, but the stops are generally bleak, barren or both. There's nothing romantically continental, or particularly world class, about them.
EDIT: Moderators, could this post be moved to the South Wales Metro thread please? Apologies for taking this thread off topic. Or if not, I'll happily delete it. Thank you.