"tubes don't really change in volume with temp,that's why they go hard (and eventually burst)."
Splitting Hairs there eh Pol?...
The Tubes themselves may not...but I'm sure you'll agree the AIR IN them does expand and decrease in volume with temperature fluxuation ( something all inflatable owners recognise) and does indeed therefore expand and contract the surface..Hyperlon...and then effect the Tube pressure,which in turn may cause stress on any surface/interface not joined to them properly.
The answer was to Beer Belly by the way..who was talking about his Osprey...although I'm sure the OP thinks just as much of his North craft!.
As to the exchange rate...yep it is a bit low at the mo,but I'm sure it's only temporary,and will be well worth it in the end!