Does the Mega packs work as replacements?

Last night I realized that, I need some add-on packs for the city plan contests city plan. As in my now lvl. 2 & 3 settlement all the plots are wrong from the designs as it's on designers choice.

But as both mega packs covers 12 different add-on, does they cover the plans needed for the contests city plans? Just to be clear that I didn't misunderstood the description on NM.
As it would only use 2 plugins instead of 12.

As a bonus question, can I update the city plan to use the correct plans after installment, without having to start from lvl. 0 again?

Do you have designers choice turned on in the Holo/MCM? It must be turned on before you start the city building. Without that plots will be chosen at random.
You can try change the plots to designers choice after they are built, but from my experience it doesn't add them all correctly and some manual changing has to be done.

The mega packs contain a selection of plots from several addons, they are not an entire replacement.
If you have both mega packs installed I recommend installing wasteland ventures also, between the 3 of them you will cover most plots in city plans as they are the most popular with designers.

If you really want a city plan to look exactly like it was designed, you can cross reference the date the plan was added on nexus with the contest archive here https://simsettlements.com/site/index.php?categories/city-plan-contest.146/
Designers list what addons they use in their entry, if they haven't DM them because the should've.

Yeah I made that designer choice were on, before building the first settlement. But as I don't have any of the packs, the game just picked random from the normal plots.

I'll take a look in the section to see what I'm missing. =)

Do you know if SS can change to the right plans for me, or would it be best to start from scratch after installing the add-on?

As I said, SS will change try change the plots but it doesn't always work properly.
Here's what to try to give it the best chance: Install the addon > while in each settlement turn designers choice off and back on again > sleep for 24 hrs > fast travel away > return and select refresh city plan and refresh all plots in the holo.
If that doesn't work you will have to change them manually. Or scrap the city plan entirely and start again.

I'll try that. Thanks for the help. =)

No problem.