Differences between Adobe 7.0 & Adobe 8

I have a pdf form I created using Adobe 7.0 professional, some in my company have upgraded to Adobe reader 8.0 and can not submit the form via email, why is this? Also if I was to redo the form in Adobe 8.0 would users with Adobe reader 7.0 still be able to submit the form? Last one if I was to download adobe 8.0 would I loose adobe 7.0 on my computer or can I have both downloaded?