Should I Repot this Phal.?

[icedtea2008]

Hello! I'm fairly new to orchids, though I have been working in a local greenhouse for two yars now, and have had plenty of time to enjoy their beauty. I received an orchid as a gift from my boyfriend some months back that he ordered through a floral shop's distributer. It was a purple viened Phal. with lots of blooms and buds, and seemed to be very healthy. The only thing I noticed was that the medium was very old - it had plenty of greenish mossy stuff and was a sphagnum/bark mix. Well for the next month or so I watered it every week plus a couple days like I read I should, but the pant started losing its flowers, and then its buds. I asked the horticulturist at my work what i was doing wrong, and he said not to worry, sometimes when they are transported alot orchids will drop their buds. Well after the plant dropped all of its buds, I decided to cut back the spike and repot the orchid in a new bark mixture. When I was repotting, I noticed almost all of the roots were mushy and the base of the plant where the roots connected was soft. this isn't good right? After trimming away all of the most obviously mushy roots and repotting it, I hoped for the best. Sadly, however, it lost 2 leaves and became loose in the joint where the leaves join the root base. It soon after died. This all happened in the span of a couple weeks. After my boyfriend told the floral department about this quick death however they generously offered to replace my orchid in full. I now have a beautiful new white showy Phal. that is a lot older with more leaves and two spikes coming off of an original spike. It too, however, has very old medium with fuzzy moss (or mold?), but noticably healthy roots mixed in with older dead ones. Should I repot this orchid even though its blooming? I hate to have the same thing happen to this one as to my last one (which I'm thinking I might have been overwatering..) Any advice?