That may not be possible, at least in the short term. Every site I've seen updated thats well ranked loses at least SOME rank for a time. If done well most recover or are even better... but it IS a risk to some extent.
I tried to: Keep all the pages and extensions the same, keep the metas, page descriptions/titles, written content, alt tags on photos... there is a lot of stuff now that I think of it.
If its ranking #1 why the hell would the owner want to screw with it, by the way? Is its conversion rate shitty? If it isn't broke why fix it?
It could very well be that the old interface is very labour intensive, meaning it would be cheaper in the long run to change the backend to wordpress. It could be that the old interface is not very secure and prone to be broken into, wheras a new backend would prevent this. Wordpress could make it a lot easier to add new modules to the website, instead of hiring a php coder every time you want to make a small change.If its ranking #1 why the hell would the owner want to screw with it, by the way? Is its conversion rate shitty? If it isn't broke why fix it?
Its a hotel website. It was coded in 2009 and therefore it really needs a facelift + new features like online booking, new pictures... I told her about the risks and "never change a running system". So you suggest to keep the content almost the same but only change the looks?