using existing stainless venting for new condensing unit

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Takagi tankless that i installed in 2009 was from Japan. Control board went out and A O Smith who bought US Takagi and does not stock replacement so i'm going to have to replace an otherwise perfectly good water heater.

I decided on a newer Rinnai condensing model that can use 2" pvc for venting "or 3" i am told". The intake from the Takagi is 3" pvc and could be fitted with 2" reducer and should work. The venting is cat 3 stainless 4" (yep it was expensive). I believe duravent makes a 4" to 2" converter for the system, but i can't get any info on whether the existing stainless venting can be used or might there be issues with the upsized diameter. I can't think what they might be, but it would save me $. Alternatively, could 2" pvc flex be run through the existing pipe and roof termination?

Since most literature concerns installing from scratch rather than replacing, i'm hoping some one has experience with that type of install
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