You can still play all the old raids & dungeons with 300 power & old weapons. Unfortunately, raids & dungeons from before Edge of Fate will not give you any sort of pinnacle or higher power gear. Their armor will also be in the old (Pre Edge of Fate) format, meaning it will not have Tiers or build archetypes ... Think of the portal as a leveling system. If you want to increase your power above 300, you need to run portal activities that push your limits. At 300 you want to select Master difficulty & add on enough challenge-modifiers to predict a potential score/grade of a B. Achieving a B will give you rewards that are a few points higher than 300. Work your way up to GrandMaster difficulty when you're closer to 400, then eventually Ultimate. You dont need to be more than 300 to run raids, but having newer amor archetypes will allow you to make better builds and stats for your raid bosses.