Originally Posted by ParamountScapin

Problem will be that you are using 10 gears where 9 used to fit. So you will have to adjust your rear derailleur so can only shift through the nine gears you want. You get to pick, though. Either drop out the first or the tenth and set the stops accordingly. You will also have to use a 10-speed chain due to the 10-speed cassette. And you are supposed to use a 10-speed big chainring with a 10-speed chain. And 10-speed jockey wheels and bolts on your RD. It will be easier to simply purchase a 9-speed cassette or two. Can generally get them new off of eBay for about $50. Actually, I just bought two of them for ~$30, each. A pretty good deal compared to $75-200 from your LBS. But. Read on.

OK - now here is what you really want to do. Send your right side Ergo lever to Tim at Branford Bike. For about $75-90, depending what parts it may need, he will convert it to 10-speed. Get a 10-speed chain and jockey wheels for your RD and, presto. Both your bikes are 10-speed. Now, that wasn't too difficult, was it. Just try doing anything remotely similar with shimaNo.

And, lastly. Tell Tim you want your 9-speed parts back. Because I want them to convert my 8-speed to 9-speed. Have fun!!