from Harbeth 30.1 to SHL5+, an upgrade or lateral move?

I wouldn't call it a lateral. The SHL5+ are more expensive and bigger. I think it really depends on the type of music you listen too. I use to owned a pair of Harbeth C7s and I was thinking about upgrading to the SHL5+ because they were a better match for my taste in music (from classical to jazz to EDM to pop to reggae), which frankly is all over the place. The 30s were never in play for me. The SHL5+ gave me that added bass that the 30s were just not able to produce. The 30s I admit sounded a bit more refined in the details but honestly though your choice of amp would make a bigger difference. The mid range on both are truly beautiful. If you’re like me in terms of music, the SHL5+ maybe a better match. What kind of music do you listen too? For me, in the long run I wanted speaker that were full range so I ended up buying a different set of speakers all together. I do miss the Harbeths. Good luck!

To me, the SHL5+ is somewhat of an outlier in the Harbeth line.  It sounds quite different from their other speakers--it has a more modern, conventional sound (not as warm, tighter sounding and punchy bass).  I like the SHL5+, even though I don't like most modern speakers, but, the only Harbeth I really LOVE is the 40.2 (very much NOT a modern sounding speaker). 

I've owned the Monitor 30s and the SHL5s and I really think it's a matter of taste.  The 5's have a bigger, fuller sound that some might describe as less "accurate" than the M30s.  The 5's are also more forgiving of lesser electronics, IMO.  Depending on your setup and room, the differences could be minimal.  Frankly, I would want a home audition before dropping the kind of money they're getting now for the 5s.  When I bought mine they were 4k.  

Unless you are ready to step up to 40.1's, I'd keep the 30.1's and get a decent subwoofer, as that will get you closer to the 40.1 sound than buying SHL5+.  I've owned the SHL5's (non + version), and am now on my second pair of 40.1's, which I love and have no intentions of selling.  The SHL5's were nice, but not in the same league as the 40.1's IME.