Not so fast with the go-around. It depends on how well the approach was planned. If you're not rushed and had time to brief the contingency and you're inside the OM then you can continue...assuming everything esle is o.k. The regs are specific:
(c) If, after passing the outer marker or
equivalent position in accordance with (a) above, the
reported RVR/visibility falls below the applicable
minimum, the approach may be continued to DA/H
or MDA/H.
So, if after passing the OM the RVR falls outside of your current category you can continue. Good airmanship assumes the contingency has been briefed. If it hasn't then go-around.
To illustrate the point, consider a normal ILS where the contingency of a failure of the Glideslope has been included in the briefing. If, once inside the outermarker, the glide slope fails, you can continue to LOC only minimums for two reasons:
1) the contingency has been briefed,
2) the regulations say you can continue because you started the approach outside the OM with sufficient RVR/vis. Once inside the OM if the weather deteriorates the regs say you can continue to minimums. The only thing that changes inside the OM with a G/S failure is that the descent is limited to the LOC only MDA. If the failure occurs outside the OM then you must go around because the requirments are different.
The final caveat is that if the contingency hasn't been briefed and all crew members aren't in the picture then you best option is always to go-around. The same thing applies to LAND 3 versus LAND 2 inside the OM. Of course, individual company ops. may give directions to follow in these cases. But strictly speaking, the regs do not disqualify the approach inside the OM simply because of a downgrading of the approach capability of the aircraft.