Question regarding SEO and website updating

Ok so i am ranking 1st/2nd on googles first place for a precent decent keyword. My website is being majorly updated. Will run on a different zencart platform (but may even have more content).

Question is, will this likely effect my positioning in a bad way?. Or does google even see this as being good?

Depends, what will be changing?

If the urls are changing, then yes that will have a impact and you must 301 redirect old urls to new ones (or better still not change the urls)