If we do commuter rail correctly, i.e. regional rail (think S-Bahn, RER, et al) then there would be no reason to change to light rail ever. We can start with European transit-style Diesel Multiple Unit trains which accelerate reasonably well and would allow for pretty fast service. Given sufficient triple tracking, all-day frequency of at least 30 min headway should be the starting point. Upgrade/increase from there. Mebane to Clayton is a 60+ mile journey - light rail would not the best mode for that anyway, it is a regional rail corridor. The BRT corridors are better candidates to be upgraded to light rail eventually.
Frequent regional rail combined with gold-level BRT is the model that the Triangle should pursue at this point.