Yes, you sure can. However, there are a few caveats:

• Be careful with the date of the change, because your plan will be prorated: https://www.koodomobile.com/en/help/proration
• And also, you will have to pick a plan that is available at the time when you wish to switch back. So if you are on a great grandfathered plan, you will not be able to pick that again - it has to be an in market plan
• If you have any add-ons specific to your plan, you will lose those.

If none of those are an objection, go for it!