I do however have several sleeping bags from Jungle to Arctic and in all but the most extreme conditions I use two sleeping bags together as this gives much more flexibility. If you have a 2 and 3 season rated bag (approx 0 and -10C) then you can…

Use the 2 season as a blanket and create a 1 season bag
Use the 3 season as a blanket and create a 1.5 season bag
Use the 2 season as normal and create a 2 season bag
Use the 2 and 3 season as blankets and create a 2.5 season bag
Use the 3 season as normal and create a 3 season bag
Use the 2 season bag as normal with the 3 season over it as a blanket and create a 3.5 season bag
Use the 3 season bag as normal with the 2 season over it as a blanket and create a 4 season bag
Use one inside the other and create a 4.5 season bag.

1 season = approx +10C, 2 season = approx 0C, 3 season = approx -10C, 4 season = approx -20C. 5 season = approx -30C.

If you’re going into extreme conditions then I’d suggest buying a bag specifically for those conditions. To be effective a sleeping bag needs to ‘loft’ and I assume an upgrade simply means stuffing more insulation into the bag, I can’t see this as being particularly effective as it will squash the insulation and reduce loft. Effective sleeping bags have box-wall or similar construction so an upgrade would mean having to virtually unpick the whole bag, refill it then restitch it. As this would need to be done by hand I wouln’t be suprised if it were cheaper to buy another bag which you can use in combination with your Mountain Hardware one.

As the MH bag has a Polorguard filling you might want your second bag to have a down filling (put the down bag inside the MH bag when in use). Your MH bag will be less efficient than the down bag but the down bag needs to be kept dry if it’s to retain it’s insulative properties.