You can pause your current or existing campaigns (non-PGN campaigns) at the same time create your PGNs, but this is NOT recommended because this will likely shock the Amazon system. Jumping to create new PGNs immediately without automating your non-PGN campaigns may result in costing you more money due as Amazon is designed to favor campaigns that have more historical data.
The sequence of actions is wrong and would likely cause you to lose money. We recommend doing a proper transition: First, you need to automate your existing campaigns and then, create your PGN campaigns. You need to allow Amazon time to associate your SKUs with the new set of keywords and build relevancy to your new PGNs.
For a complete step-by-step discussion, see FAQ: Do you recommend starting fresh with new PGNs and discontinue my existing campaigns