You are asking this question, which means you already know and I am presuming retaining the same URLs is not a possibility. Even if it's a slim possibly, I'd suggest looking into it further. If not, then as long as you can 301 each and every URL to the new target URL, you should be fine. I have seen a +/- 10% Dip in traffic and rankings (if any) and it all varies on the authority and strength of your website. If you say, you are ranking well with some traffic, it definitely has some strength and I would not worry about moving it as long as you are prepared for it, in terms of redirects and maintaining similar or better On-page SEO and Internal Link Juice. Typically, when CMS's change, so does the internal link juice etc and that can have an affect even if you do the redirects fine. So you need to do an Audit of what's before and after, how optimized is the after compared to the before and what does your competition look like (if any).
I hope this helps.
