Absolutely! Upgrade/downgrade your server at any time and pay onlt the difference at a prorated amount. Upgrades can be done automatically in the client area.
You can 100% use mods on your server make sure you have the right plan to run them for minecrfat we recoment 5-32GB depending on what you whant to run.
Download the files or backup then un archive it on your pc then make a zip file and upload it using SFTP depending on size of file then unarchive in panel if need help open a ticket.