You can upgrade from one major version of Business Central to the next. However, minor updates are regularly made available for each major release, like 18.11 or 19.6. Whether you’re upgrading from one on-premises version to another, or you’re migrating to Business Central online, it’s important to target an update versions that are compatible with your current version.
Control Add-ins written in C# are not cross-version compatible. You cannot use an assembly compiled for 2013 R2 with 2015 (or the opposite way around, or any other version combination for that matter) without recompiling it with correct extensibility framework assembly reference.