I'm using a Meridian 506.20 now, which is very detail and nice sound... but at the same time, very revealing too.
For some reason I missed a lot the Rega Planet that I had a couple of years ago. I missed the laid back analog like sound. My system was diff. at the time (although the Mac MC2105 amp and the Piega speaker is still the same) but the Planet was very smooth and warm.
I have an opportunity to purchase a Planet 2000 now. By selling the Meridian I might even have some left for a nice DAC like the MSB Link III or Beresford in the used market for the iTunes music in my other system.
But am I downgrading? I know I shouldn't care... but it's bothering me.
If you can buy the Planet and listen for awhile before you sell the Meridian you will answer your own questions. If you really enjoyed the planet and miss it's sound then no one here can tell you whether it's a downgrade for you. In the end they are your ears and the music you listen to has to engage you and move you.
The 506.20 was one of Meridian's more successful endeavours in my view. I always found it more engaging than the 508.24, and a bit more cultured than the earlier 506.18.
It's odd to point at its revealing nature as a downside unless perhaps this is getting in the way of enjoyment. This can happen when the presentation highlights the pieces leading to a less cohesive result which becomes distracting and makes you less engaged. This effect can happen in many ways in my experience. I've found that adding a DAC to a system can easily do this. Unsympathetic pairings of kit or cabling can also make it occur.
From a purely subjective 1:1 view, I would suggest that going from a 506.20 to a Planet 2000 would normally constitute a downgrade. The Planet has far less resolution, is not normally as engaging and sounds a bit slow by comparison to a 506.20 in a sympathetic system.
However, you could have some unfortunate mismatch in your system currently and this may be removed with the Planet which could give you a beneficial result. The fact is we can't tell from here in my view, especially without knowing your system components.