Upgrading air units

[LGKMAS]

Could some kind soul take pity on an old man and point me to the place in the manual that allows you to change the upgrade path of air units? Alternatively, if the manual does not specifically address the matter, could I have some threads that discuss the matter?
I have some units that seem to say they will upgrade to what I consider an inferior aircraft. And some members here have suggested Stateside 4th AF units should be downgraded in aircraft to allow the more advanced airframes to go to front line units.
I have been unable to discover how to do this. I think PP are required as every time I click on the upgrade yellow text of the air unit, there is a text message saying so many PP are available but I have not found out how to use them yo change upgrade paths.

[Alfred]

So many factors at play, none narrowed down by the data provided to date.

Page 148 of the manual provides some basic information on aircraft upgrades.

Many forum threads on the subject. This one might be the closest to your problem.

https://www.matrixgames.com/forums/tm.a ... e&#4449009.

Provision of screenshots will be the quickest way to understand your actual problem.

Alfred

[LGKMAS]

Thanks Alfred
I looked at my settings and discovered that from the start I have had PDU Off. That would explain why I now cannot change the upgrade path.
I suppose, 200 odd turns ago, and in my first game, I decided to keep things simple while I learnt a bit more about how things worked. I might have to look more closely when I have time at what else I switched on or off to supposedly make it easy to learn.
Ah well, live and learn

[Alfred]

Further to my previous post, s.16.2.3 of the manual deals in more detail with aircraft upgrades than does page 148.

Alfred