How do I get "promoted" as a PC member in a conference?
I am not sure if this is the same in your field, but in mine (software systems) PC member and senior PC member are actually different roles with different tasks. It's not so much an honour badge to be a senior PC member than being assigned for a different job in the selection process (typically, PC members do most of the reviewing while senior members do less reviewing and more "meta-reviewing", similar to an Associate Editor for a journal). It's also not the case that, once "promoted", you will always remain in that role - every team of program chairs is free to assemble their own PC, and they invite people to the different roles. This often, but not always, includes re-inviting previous committee members.

I know of cases where people who used to be senior PC members were asked to serve as regular PC members in the following year (or not at all). Also, many senior PC members are actually more junior (in terms of academic age or rank) than some of the regular PC members. This has to do with PC composition - you want to have good coverage of all topical areas that you receive contributions in, and of course past performance and the subjective impression (of the PC chairs) of the different people plays a big role as well.



That is all to say that there typically is no defined promotion process between different levels in a program committee. You need to do good work in previous years and gain sufficient notoriety in the community that the next batch of PC chairs sees you as somebody who they trust to oversee the selection of papers in a specific niche topical area.