After an afternoon oldie powernap, curiosity aroused, I started mucking around (is that a comprehendible phrase in your part of the world?) with "Mr. Postman" as taught by Caren.

A few things I discovered which might be of interest to or benefit you.

Keys to that change from open D to barre Bm. It is more critically timing dependent in order to form the barre formation and minor in the available time, does require a lift off and 100% instinctive barre Bm finger placement. It is the only arguably challenging change in the riff, and is dependent upon proficiency with Bm. I play a lot of The Beatles, so Bm is a stock in trade chord for me. What will get you over the line with it in this riff is changing from D to Bm over, and over and over, and over until you just get the timing of the the lift off and landing including the fingering and clean notes ringing from each string in the barre chord for tone right.

Now if you're frustrated and just want to play the song whilst you're working on that, it's much easier to play the riff in the same key using all barre chords. If you can play it on either an amplified acoustic that way, or an electric, it sounds even better accompanying voice IMO. The all barre progression from D to Bm is even easier than from open D, as are the progressions repectively to barres G, A and back to D. Just food for thought. You can play in key along with Caren that way while working on your open D to barre Bm competency.

Another alternative if you just want to play the song, again while still working concurrently all the time on that open D to barre Bm challenge, is to play "Mr. Postman" transposed into either the keys of G or C. It is much easier as the progressions of the riff in those keys can be all open chords if you want them to be, so they flow easily off the fingertips as well as the musical staff to work well either way. I just tried it. My Mrs. thought they all sounded pretty cool. She was tapping and singing along. A lot like "Hallelujah" which works in multiple keys (I play it mostly in C) or "Love is All Around" which works equally well in the key of G as its original The Troggs key of D.

Remember to keep the fun in it all. You will get there. 👍😄