I thought it was changing, hence the three tiers, but that may be a mistake on my part.
@nick_h. To move up you need to apply to become a WT team. Presumably, this is measured on financial ("do you have the budget to pay the riders?") and sporting ("Is your proposed team good enough?") criteria.
I was reading yesterday that fourteen teams have applied for WT status for 2022. The nine existing teams and five new applicants, Rally Cycling, Uno-X, Jumbo-Visma, Team Tibco-Silicon Valley Bank and a new team, Roland Cogeas Edelweiss Squad. It's not clear how many WT teams there will be though, i.e. is there an upper limit as per the men's regulations.