Releases & delivery: music distribution help

From submission to going live on stores. How long until my release goes live? Why does my music sound different on Spotify or Apple Music? Can I edit a release after submitting? What if a store rejects my release? Can I switch from another distributor and keep my UPC/ISRC? Why does accurate metadata matter? Will my catalog have DSP downtime when I migrate? What about pending royalty at the old distributor? Do I need to keep my old distributor account? Does Spotify for Artists get affected by migration? What’s the migration fee? My catalog is large (>100 releases) — bulk support?