If you downgrade to a plan with lower publishing limits your published infographics stay published even if they go above the limit. However the oldest published infographics above the limit are not editable. If over the limit, new infographics can't be created, and unpublished infographics from the higher plan cannot be published until enough infographics have been removed from your library to go under the limit of the basic plan.
For example, you have downgraded from a Pro plan to a Basic plan. You had 40 infographics published with the Pro plan. Now all the 40 infographics are public, but you can edit only the last 10, as that's the limit for the Basic plan. If you want to edit one of the other 30 infographics, you have to upgrade to a plan with higher limits or delete some infographics.
The same happens with features that are not available to lower plans. Infographics with features that are not available in a lower plan remain published, but you cannot edit an object using features that are no longer available to you. To edit an object of such kind you have to upgrade to the plan that enables the feature. However you can delete and move objects regardless.
For example: you have downgraded from a Pro plan to a Basic plan. You want to edit a regional map, but it is locked for editing as the feature is not available on a Basic plan. You can delete the map or rearrange it within your infographic. If you want to change the data on the map, you need to upgrade to a higher plan.
If you downgrade to a lower plan, you retain the use of as many uploaded images as the new plan permits. The older uploaded images that go above the limit will be locked for use.
For example, if you downgrade to Basic, and already have 40 images in the gallery, you retain the use of the last 10 images you uploaded, and the other 30 are locked. Since the limit has been reached, you cannot upload more. In order to upload more, you either have to remove images until your image count is under the limit of the current plan, or upgrade back into a higher plan.