Minimum amount of RNA for cDNA conversion and qPCR
Two answers follow:


Technical experience of many: it should be no problem to have the same protocol with a twice-lower starting concentration, without making any adjustments whatsoever. The suggested 1ug is very much on the safer side of things, 2-fold fewer transcripts makes almost no difference. Maybe at 1 or 2 orders of magnitude (10 to 100s) less will you run into problems or a need to correct your reactions. 
Scientific orthodoxy: the most scientific piece of advice anyone can offer you would be to try it both ways and compare your results using qRT-PCR. I'm guessing both approaches will show very similar results.