Big changes are usually a bad thing. They’re expensive (both time and money), members don’t like them and it’s difficult to predict their impact upon the community.
Sometimes, the logic is just wrong. New platforms can guide the actions of members once they visit the community, but it can’t attract new people to the platform in the first place. If not many people are visiting the platform, then a new platform wont solve that. This depends entirely upon your growth efforts.
If your community is failing, it’s not usually because of the platform. If your community is succeeding, then why change the platform?
Instead of changing the platform, aim to continually refine it. We prefer continuous tweaks. We’ve covered many of these before (link).
For example, very simple changes can have a big impact. Changing the length and copy of the confirmation e-mail, repositioning the registration banner, or tweaking the notification messages can have a far bigger impact than a new platform.
Another example, if you get rid of the large banner graph at the top of the page an replace it with the latest discussions in the community, you will find the level of activity rapidly increases.
Spend the rest of the budget on more and better community mangers. Just please, stop spending so much money on expensive platforms. Spend the money on finding the best people to manage them.