Yes, you can connect a new/different Facebook account to your Post Planner app anytime.
Please keep the following in mind:
The new Facebook profile should be an admin of the same FB pages and groups if you want any planned posts you have for those pages and groups to continue to publish successfully from Post Planner
Nothing in your CONTENT section will be affected
To connect a new Facebook profile to your Post Planner app, please do the following:
(1) Login to the FB account you want to remove from Post Planner
(2) Go to your Facebook account settings here and remove Post Planner:
(3) Logout of the FB account you've just removed Post Planner from
(4) Login to the new FB account you want to connect Post Planner to
(6) You should see all the pages and groups your new FB account is an admin of
(7) Click to activate (add green checks to) the pages/groups you want to post to from Post Planner
You should be all set!