You'll be "the level you have obtained" which is 20. Since gear no longer affects your level, you will be level 20 player level. If you're wearing 42 light gear, your power level will be the same as it is now when you are 34. You aren't losing anything. Activities will now have recommended Light amounts, not player level. So level 35 PoE will have a recommended Light of 168, not recommended level 34.