Re: can changes be made on community edition be transferred to enterprise version easily?
I'll try and answer your questions the same way you asked them
You can migrade your code to Enterprise Edition pretty easly, however there are some areas of the solution that are different and use different Blocks and Models that you may have to adapt your code to match. Also Enterprise edition comes with its own base template that you should properly build from if you want the full functionality of Enterprise Edition.
All the code changes you made in Community edition shouldn't be within the Magento Core, so you can move it independed of what edition you are using. Again you might run into some problems becaouse Enterprise Edition are using different objects in some part of the system.
I'm not really sure how much I can talk about this without showing the code, but Enterprise Edition is basicly a lot more power user / enterprise functionalitet and some areas of the system completely rewriting.