I use Kinetic K-Gear software to see what difference changing gear ratios is going to have.
Does going to a 11-36 make that much difference to using an 11-34???
Depends for a start on what size ring your using up front – if your planning to use a 32 up front you lose your three lowest gears at the back, if you go to an 11-36 you only lose the last two.
If you follow fashion and go for a 34 up front your back to losing the lowest three at the back but gain one bigger gear up front… If it was me, and I had an 11-34 9spd cassette kicking about I would simply give 1×9 a whirl with what I had. If I only had a 11-32 cassette to try I would try it, and beg/borrow an 11-34 off a mate to give it a go.
Changing to 10 speed or even buying new bits at all to “try something out”, when that something can almost be accomplished without the expense seems a little daft.