how will this circulator work?

There's more variation in lines you can get, but not to the extent that it was originally (I got like 19%~23% Buff Duration at Epic). And as far as I know, it's the same as now; your main line is of the rank your inner ability is stated to be, the bottom two are usually crap. I locked at epic for a while since it was pretty cheap, but I didn't notice any epic lines other than the first one. I'm also pretty sure you can't rank up when you lock your rank.