Ensure your new PGN is bidding just a bit higher than your aged campaign.
Monitor your PGN and existing campaigns
From Day 1 to 5 Let your new PGN run for 5 days. Running it for 5 days would not be enough to generate metrics but 5 days would allow your PGN to show up and age gradually.
Let's allow Amazon to detect your keywords and targets.
During this first few days do not expect any changes in your campaign metrics.
From Day 6 to 10
When the PGN starts showing:
Gradually decrease the Max. Bid. of your old campaign. Having a daily reduction of 10% on your Max. Bid should be fine.
Allow your new PGN to run as is, no changes, to attract impressions and clicks.
After Day 10
Reassess your PGN. Check your stats: impression, clicks or sales. Are they better?By Day 10, at this initial stage, the PGN is "alive". The SKU is getting associated with the keywords submitted during PGN creation. Your PGN is expected to bring most of the impressions or clicks.
Day 11 to 21
You can now safely shift the ad spend from the old campaign to the new PGN. The shift is done gradually to avoid shocking the Amazon system, as well as you allowing Amazon time to associate your SKU with the new set of keywords, and build relevancy coming from your new PGN.
Keep decreasing the daily 10% Max. Bid of your old campaign for 10 days (from Day 11 to Day 21).
Keep your PGN as is. Do not implement any changes.
Day 22 onwards Review your PGN.
When all the key metrics are looking fine, then Pause your old campaigns.