It’s worth noting that some dioceses require +/-6 years of seminary formation (assuming you are discerning the diocesan priesthood), regardless of previous philosophy studies (including a BA). My understanding is that this is because the “formation” aspect of all years of seminary cannot be replaced by just fulfilling the academic requirement of philosophy.
Seminarians with a previous philosophy BA, though, might spend their “pre-theology” years studying for a MA in philosophy, studying Latin/Greek, etc. I imagine this would be decided on a case by case basis.
The best thing you can do is speak with your diocese’s vocations director!
Of course, presuming you will attend college before seminary, I hope this doesn’t deter you from pursuing a BA in philosophy (or philosophy/theology double major) - it is the best thing you can do to better prepare you for graduate level theology/philosophy studies!