For users already logged in they should see the price change immediately.

Do you mean it's dynamic only for logged in users ?
if yes how come it cannot be so for not logged in ones ?

It maybe easier to just have a plan per N1-4 and condition those plans to show based off that fields value. This is jQuery powered on registration so the plans displayed can change dynamically.

In fact we have 5 plans with annual and monthly payments, which would make 10 different plans to maintain.
This is why the option to only change one annual and one monthly was much better.
The thing is that we have a lot of integrations and interactions in the plan and multiplying them multiplies the potential errors.
Would it be possible to implement a price change at registration depending upon a drop down fied ?
Do you have other suggestions ?

You will not be able to see the price change at registration. Once they reach the basket page then the final price will be seen as the conditions for the promotions will take affect.

This may be a solution :
Can we get the basket page before all the information tab ? so that the customer will check the price before registering
Can we have a dynamic text in the plan depending upon the drop down field ? so that it's not a price change but only a text change explaining

Could I add an auto action code with jquery checking the dropdown field chosen on registration and showing the correct price on the CBsubs tab ?
If yes what would be the trigger to use ?

I am following on this.
As we absolutely want to avoid to maintain 10 plans, auto action and jquery seem the best solution to show a text plan price while masking it to guests, and then leave the system compute the right price for the basket.

However, we tried to use afterdrawplan but the action writes the jquery code only to the first plan.
Can you suggest another trigger ? we want to write the jquery code to three plans to display three different prices