I agree with dahermit. The loads you are intending to work-up will not be too much for once-fired cases. And, if one of those cases has a defect, firing it once will probably reveal that. So, no down-side to firing factory loads for the brass.
With respect to your own suggestion: why would you decap the cases? Just work-up the loads with the factory primers. You should resise the case mouths after removing the bullets, which you can do without decapping by removing the decapping stem from the sizing die.