Yes, it is definitely possible to downgrade from Plesk 100 domains license to Plesk 10 domains license. However, if you downgrade from Plesk 100 domains to Plesk 10 domains then you should make sure that you will be able to host only 10 domains with Plesk 10 domains license
Also, when it comes to downgrade, please check with the web hosting company if they provide Plesk 10 domains license with the hosting package you have. It might be possible that they do not provide Plesk 10 domains license with the hosting package you have :( If this is the case then you will have to consider Plesk 30 domains license and you will be able to host 30 domains with it
