Are you wanting to
( 1) upgrade the sub domain with jD 1.8.4 to jD 3.2.51
(2) transfer the Downloads in the jD 1.8.4 to the sub domain with jD 3.2.51 ?
If (1) then your jD 1.8.4 sub domain is presumably on Joomla! 1.5 so that would involve migrating that site to Joomla! 3.2 and then upgrading jD to jD 18.104.22.168
If (2) then it may be simpler and much less effort to transfer the files, a few at a time, and use manual monitoring (turn auto monitoring off). The viability of that approach is dependent on the number of Downloads involved. If it is just a few hundred then it is worth considering this less 'glamorous' but pragmatic route.
So please advise how many downloads are involved and if you have made extensive use of descriptions, images, and what types of file.