Hi Samwise, yes, this is possible. Just place all scenarios in your Game Content/Scenarios folder.
You need to thoroughly re-test all of your scenarios though. Some new bugs may sneak in when you transfer your files.
Do refresh all
victory conditions for the human and all computer players. They may get messed up.
If you have long introduction texts, best do not touch them with the DE editor. There is a bug that makes portions of text disappear, as soon as you edit it anywhere (e.g. you correct a typo in the beginning and the last third of your text just vanishes forever). The bug is less likely to happen with short texts, you can edit those. Until this bug is fixed, best do not edit your old long texts in any way at all. If you absolutely want to try it, make a screenshot of your old texts first. You cannot copy and paste them unfortunately.
If you had bitmaps with your scenarios, they will not show up anymore in DE, unfortunately. However, AI and PER files will still be saved in your scenarios.
If you have any special gameplay features that relied on 'quirks' of the old game (e.g. bird-hunting, boarding a transport through walls), they may no longer exist.
Some terrain objects such as cacti or grass clumps cannot be deleted with the Delete button. Placing and then deleting a building over them helps though.
Besides its bugs, the editor also has a lot of cool new features, e.g. new terrains and new map items, and it's fun to design with these. I wrote an article in July 2018 about the DE editor. Some bugs mentioned in that article have been fixed, but the rest of the article is still up to date: http://aoe.heavengames.com/siegeworkshop/map-design-tricks-for-aoe-definitive-edition/
Hope this helps.
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