I want to book an award flight on LH using my UA MileagePlus miles. The onward flight is available in J, but the return currently shows only in Y. If I book it in Y using a saver award, and if the J award becomes available in the future, does UA charge any change fees to upgrade to the J cabin on the same flights? It also seems LH doesn't make J awards available to partners more than a few weeks in advance? Are there any tips to get LH J flights using UA miles several months in advance? Thank you.

PS. I searched this forum and the internet but couldn't find an answer.