Why should you change? Because OS9 is a blind alley, a dead end, the end of the evolution of that code base.
Sure you can stick with it but you're going nowhere, there will be no more OS9 upgrades, the rest of the Mac world will leave you dead in the dust in a few months as OSX becomes the only bootable OS that new Mac hardware will support.
OSX is the future and the future is now, you need to get up to speed with it and you need to do it asap before the learning curve gets steeper. When OSX came out I downloaded all those little utils that make it more like OS9, today I'm not using a single one and having to use a machine with OS9 is so alien it's as bad as using Windows. Hell I don't even have OS9 on my PowerBook anymore.
It's Quark and their ilk that need a solid kick up the backside for dragging their feet over carbonizing Xpress and breaking the Apple programming guidelines so their stuff runs poorly in Classic.
I'm an IT manager for a design and manufacturing company, we have 15 G4 towers. I have OSX loaded on every single Mac that will support it and I have my users getting used to running it 24/7 and only booting back into OS9 to run Quark, and soon that will be over as we move to InDesign because Quark won't get it's act together fast enough. We DO have bureaus that will take InDesign files and they will be getting our business.
Evolution? It's a tough thing for some to accept but for those who do the future is bright, very bright indeed.