Hello @Nicolas ,
Yes, you can upgrade from Standard to Enterprise or Premium and vice versa. Upgrading the tier is strictly based on your requirement meaning , you can upgrade from standard to enterprise or premium, if you have a need for using resource forest. And depending upon the number of resource forests , you can decide the SKU. Downgrading the tier is possible when you are not using a feature which the higher SKU tier SKU permits which means if you are using resource forest feature which comes with enterprise /premium SKUs, you would not be able to downgrade as long as you are still using the feature. In case you want to downgrade the SKU to a lower tier, you would need to remove all the resource forest trusts and any feature exclusively available at higher tiers before you could downgrade to standard.
Normally the Standard edition is like a normal AAD domain services instance where an organization does not have a need to interface with On-premise directory. Once you have the need to create a trust , you can choose the SKU as shown in the below screenshot depending on your needs. You can check the pricing page for capacity planning within your environment.
I hope the above clarifies your queries. Should the information in the post helps you , please do accept it as answer so that it can help other members of the community. In case you have any further queries , please let us know and we will be happy to help.