The post was only made for spammy reasons, but I liked what it said, so here it is quoted:
The reason is because when you will update the design, there will be some down time. The other thing, there will be some changes in URL as well.. If your client’s current website is in asp then getting it converted into WP will change the links somehow..
Even if the previous version is in php then also there will be some changes in site ranking. This can be because the link navigation will be different. The site loading time will vary for sure.
I had this case before. Even i didn’t made any changes but just bought a website from my friend. It was transferred and was active within couple of hours and still i lost the ranking of that website on some keywords..