Hi,
I can only speak from my own experience on this. Whilst a 301 redirect is fine from resolving the change of URL issue I have found that Google in particular is very sensitive to other site design changes.

I have done a site wide cosmetic makeover on several sites over the years, one quite recently and whilst the site content was not changed the templates used to create the site pages were redesigned.

The result of this site wide makeover was an almost immediate drop in ranking for the keyword phrases that the site use to rank well for.

My explanation for this, since it happened on several sites is that the site makeover even if it was only the template design was flagged by Google.

It caused Google to do a complete reassessment of ranking for the entire site. Whilst the site was back in the melting pot the sites ranking disappeared for several months and in some cases did not reappear.

My advice if you are changing the site templates is do it slowly and in stages and keep an eye on the sites rankings. If you trigger Google to come do a site wide reassessment you could find your rankings do drop.

Andy
Search Engine Optimization Specalist - Become Your Own