Originally Posted by demue

The OP is staying 2 nights I gather from Mon evening to Wed morning so that's why OP is mentioning a full day in Tokyo. Definitely IC Strings being next to JR Shinagawa which is a NEX stop makes sense.

As Platinum, an upgrade is unlikely, but if lucky OP may get a better view or a slightly bigger King Classic or so. Best is to expect nothing and be surprised. Whether Ambassador is worth it is a different question, for OPs case I don't think so unless there is a good use for the AMB free weekend night certificate during the membership year.

We are at the property right now and as Diamond Ambassador, being a repeat guest though and on a 4 nights corporate rate stay, we got very lucky and were upgraded to a One Bedroom King Suite from a 1 King Classic room. This is a multi level upgrade that the hotel usually doesn't give even to RAs. No Club lounge benefit's though of course, but in Tokyo there is so much good food that it is not needed at all. YMMV.