I started playing the trumpet last September after a 35 years.
A Conn 22b made in 1930, the horn sounded great my tone and range came back, everythings great.
Five months ago I purchased a cornet on ebay that I am very happy with,
it sounds great, it's fun to play. I have been playing the cornet and not the trumpet since the purchase.
A few weeks ago I picked up my trumpet and now it sounds sour to me.
I used to have great tone (too me) and now it's sour!
I suspect that if I put the cornet down and exclusively play the trumpet
I'll get good tone quality back.
Anyone experience this?

Yes - I suspect that we have ALL had this experience. The cornet mouthpiece is likely significantly different from your trumpet MP and, of course, cornets sound somewhat different in any case. So, your lips (and your brain and your ear) have adjusted to the new setup and now the trumpet doesn't sound the same. I have found that by spending a certain amount of practice time on each - not necessarily every day but a couple of times a week, I can stay up with the various instruments and keep them each sounding - more or less given my progress level - in tune and my ear accustomed to the tone.

By the way - when you say that you started after 35 years - does that mean 35 years of thinking about it or restarting 35 years after you last played? Either way, welcome to the "Senior Contingent" of the forum. We all share a lot of insights and experiences.

I agree with Come Back Kid. Although the differences seem subtle, it is important to practice on both if you're going to play both. I started my comeback on an Olds Ambassador Cornet and had similar difficulty getting switched back over to trumpet. The tone just didn't sound right, and it seemed very hard to fill the trumpet with air. Now my practice routine consists of mostly trumpet, followed by cornet, and finally a few minutes on the C trumpet most days of the week. Aside from the usual struggle to get better in general, I'm pleased with my sound on all three now.

Gord,
the reason that you experience this is that the playing system is not just breathing and chops, but also the sound in the room coming back to your ears and then to the brain.

When we first blow into the trumpet, we are playing on "autopilot". The sound comes out of the bell and the reverberation of the room "echos back to our ears/brain, we then make adjustments unconciously. We are then content (when things are as expected) or confused (when things are different).

Playing the cornet has trained your brain to expect a certain tone for a given "blow". When the brain does not get what it expects, the reaction can be pretty strong. This is why a trumpet/cornet feels more free blowing in a nice sounding room and stuffy outdoors or in a practice cell.

I would not switch too much until you get your chops back. Once you have enough strength, practice both and you will be content with both.