O2 sensor selection...

[Littlemow]

Back in 2011 a previous owner of my 1999 Boxster replaced the bad 2.5 with a 2.7 from a 2001 Boxster.  It appears to be a professional installation (or at least carefully done), and it has around 35,000 miles on it since then.  The ECU has also been reprogrammed (by ECU doctors) for the 2.7.  However, it smells like it is running rich and I fear will start fouling out spark plugs.  I'm planning to replace the two upstream O2 sensors because those are responsible for controlling the air+fuel mixture.  My question is whether I should buy O2 sensors for a 1999 Boxster with a 2.5, or a 2001 Boxster with a 2.7.  I've checked a couple sources and the part numbers given for each are slightly different.  
What is community thinking on this?

[JFP in PA]

Welcom to RennTech 

 

Before considering replacing ANYTHING, first do some diagnostics: Using a Porsche specific scan tool, check for any pending codes, check all four O2 sensors for function and voltage readings, check the MAF for voltage readings, and run A/F ratios; then and only then will you have the information about what, if anything, needs to be addressed.