I hate to burst your bubble, but I do NOT advise this strategy.
Here's why: your garment will be the wrong length and your sleeves won't fit into your armholes.
Let's use an example: Cheesy Puffs from an early issue of Knitty. There are four sizes - M, L, XL, XXL.
Let's say I want to make the L size, with a finished chest of 48 inches. And I can't get the called-for gauge of 13 sts in 4 inches, I'm getting 14 sts instead. This means that my fabric is smaller than it should be.
So, I think to myself: I'll just knit the larger size.... that will work out ok, won't it? It's only one stitch and one size, how much trouble could it be?
You'd be surprised.
This one is pretty easy to fix, but it does require some thinking and planning ahead. It requires you to read through your pattern and check all the various lengths, compare them for the two sizes, and adjust the XL sizes to use the L lengths.
Problem #2) Your sleeves won't fit into the armholes.
Again, the armholes are given by distance: for the size L, you have to work 11 inches after the armhole shaping. And the top of the sleeve is calculated carefully to be 22 inches across - twice the depth of the armhole. For size L, the sleeve is 73 sts at the top, at 13 sts/4 inches. (And 13 sts/4 inches = 3.25 sts/inch.) Since the sleeve is worked flat, take off the 2 sts you'll lose in your seam, so that gives you 71 sts. And 71 sts/3.25 sts per inch = 21.9 inches.
But if I work the size larger, I've got 75 sts at the top of the sleeve, at my gauge of 14 sts/4 inches. (And 14 sts/4 inches = 3.5 sts/inch.) Take off 2 for the seam, which gives me 73 sts. And 73 sts/3.5 sts per inch = 21.4 inches.
But if I've been following the instructions for XL, then I've been told to work 12 inches before the armhole shaping, so now I'm trying to fit a 21.4 inch sleeve into a 24 inch armhole.
Can you fix it? Sure, but at that point you're basically redesigning the garment. You'd need to adjust the depth you work for the armhole (11 inches instead of 12, that's not too hard), but then you'd still need recalculate the number of sts you need for the top of the sleeve so that it fits that depth, since the 75 I'd been working with is still too small for the 22 inches. I'd want 22 inches * 3.5 sts/inch = 77 sts. Which means that I'd need to recalculate the increases for the sleeve - since working an extra repeat of the 4-row increase pattern on the sleeve may well make the sleeve too long... and so it goes.
And this is for a straight armhole. The situation only gets messier for a shaped armhole and a set-in sleeve.
Surely just knitting another swatch is easier?