Purchasing Rev C

I realise that the details of purchasing a Rev C are down to the way
Digikey handle it, but will it be obvious that it's a Rev C we are
buying?
I think there's going to be a small price increase, so is that the
best way to tell?
Will Digikey be selling old stock alongside Rev C boards?

I realise that the details of purchasing a Rev C are down to the way
Digikey handle it, but will it be obvious that it's a Rev C we are
buying?

It will be obvious.

I think there's going to be a small price increase, so is that the
best way to tell?

Yes, but there will likely also be a different part number.

Will Digikey be selling old stock alongside Rev C boards?

Only until the Rev B boards run out.