Suzuki 2 Strokes

Folks,
Still running in my GT550B after a rebuild last autumn. Everything is nice but for one thing and I noticed it last ride, but never previously: gear changing downwards while riding normally now requires a noticeably higher pedal pressure than before. Changing up is Suzuki's usual silky ease; changing up and down while sitting with the motor idling likewise, even downwards, no issues. Only when riding and changing down, it feels 'notchy'.
I never had gear issues before, and when rebuilding it I did not disturb anything about gear trains and forks.
Anyone got any ideas as to how this has appeared and what I might try to improve it? I can't think I have suddenly got a bent selector fork and my records don't mention odd gear change last year since the motor was buttoned up. I did ride it 25miles right after the rebuild (blown fuses stopped me doing more and I only recently found and fixed the short - trapped wire in headlight!) and gear change was fine then, Oct 2015.
I've checked the chain: its not tight.
Clutch cable adjustment is such as to allow full disengagement with only a bit of lever (I adjusted it after the last ride like this, but its not made any difference) so its not cable.
The gear oil is the same stuff I've used in all GTs and this bike specifically (Halfords green).
The only thing I did do was to top up the oil after the first, fuse blowing test rides last year, because I noticed a bit of gear whine and I think I over did it by adding 300ml before it came out of the level hole. I've taken that bolt out and letting it drizzle overnight. I'm hoping an overfilled box is 'it'. Does this make any sense to anyone? all ideas *welcome.
Cheers,
Mike
*apart from messages of sympathy and kind offer to take the bike off my hands for a fiver

Just test ridden after allowing the gear box oil to find its level overnight (quite a bit did come out so clearly I had overfilled it, perhaps by as much as 300ml!) and before I rode it, I tested it on its stand. Gear shifting was then fine down as well as up in gear with the rear wheel turning. I did also go through clutch lifter and cable setting but don't think I had it awry.
Good news is its now changing gear sweetly again, phew! it probably was too much gear oil, who knew?
I'm well over 100miles on the new engine and everything seems peachy so far, so I'm allowing myself to be more optimistic.
Mike

Addendum: another GT owner in UK reported they'd had the same notchy down changing issue also due to an overfilled gearbox, resolved by draining and refilling to the level. Interesting!