Based on my experience:
Both versions will be supported as long as possible. Trying to support 7 until it reaches end of life, only dropping support if versions are really get incompatible which should not be the case because of SCL. And there will be a wiki entry helping with migration which will be smooth based on all migrations I did in the past.
Our desire is to support El7 and EL8 for 2-3 releases in parallel and then drop EL7 to reduce our infrastructure overhead. There will be migration paths and tools to help move from EL7 to EL8 and that journey will depend on what set of plugins you have available.