Will transferring a current website to Squarespace affect site rankings?

I've got a client who is considering moving her website from Weebly to Squarespace but she is concerned that she might lose her ranking as the site has been around for many years so it is high up in the search engines for her key words. Does anyone know if transferring a site to another platform will affect rankings? I'd hate for her to lose business.

Even if the content is exactly the same from the old site to the new site, moving from one platform to another will always generate a small decline (10 to 15% are rough estimates) in SEO and traffic performance for a month or so. Best way I can describe it is that Google sees the code changes on each page and takes a short step back to reassess everything. It usually only temporary and if the benefits to moving off Weebly to Squarespace are big enough then its usually worth it.

You will want to check out URL redirects.

Amen to modsquare's response.

Amen to creedon's response - so, just make sure that they keep the exact URLs so as to minimize having to specify redirects.

Thanks so much for your advice, that will help me explain it to my client.