Yea I agree if everything is kept the same you shouldn't see much difference but you should expect a drop and prepare for this.
That said you need to ensure the new CMS is capable of accommodation the changes, again be well prepared.
If you intent to make changes then ensure you follow all the necessary steps and procedures that come with a site migration.
As it effected the ranking last time you can be sure it will happen again, you might want to ask yourself why your wanting to change content management systems and weight up the pros and cons, I have a site on CMSMS and I personally don't like it but each to their own 