Legacy to Java isn't possible while Legacy to Bedrock is simple enough or Bedrock to Java with DragonProxy.
If you want your 360 world to work I'd say update it to Bedrock first then use DragonProxy to allow it to work on Java.
I've never tested this myself but as far as I know this is all you can do for world conversion between versions of the game. I just happened to hear about DragonProxy a while ago and keep the link on hand.
DragonProxy is for Bedrock to Java, think Windows 10 Edition to Java Edition (which this is more so suited to but I think it could work for Bedrock on other platforms if the files are the same for all Bedrock versions and you just have a USB [preferred] or maybe Cloud Access).
My guess is you'll want to 1.convert your 360 world to the Xbox One Edition (if you don't have XOE then this makes things difficult), 2.I'd recommend getting all worlds you want to Xbox One Edition, 3.if you have the Xbox One Edition with those worlds then Bedrock is just a thing you download you don't have to pay for Bedrock if you have a Bedrock compatible platform like the Xbox One, Switch, PS4 recently, Pocket Edition or Windows 10, 4.XOE worlds to Bedrock then to your PC via DragonProxy to Java.
It's this complex as the 360 and legacy edition worlds in general don't use the same files I'd assume like the Bedrock ones which is why Bedrock makes things much easier (for development I know this to be true but my guess is the file formats for Bedrock are more accessible).
My guess from here is to get the world files off your Xbox One to your PC and use DragonProxy. This is pure speculation however I've not tried this myself but from research this is the likely option to for over others.
Well that does make things awkward. I don't want to say go out and get one just for this but I don't see any other way around it. The Xbox One onwards method is the most simplest and official way to upgrade/make worlds compatible (besides DragonProxy being a third party converter)
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