How would they pack the court if no conservatives retire? I’m pretty sure they all have decided they can last another four years.
Expand the court to more justices (the law currently sets it at 9 although there is no constitutional upper limit) and simply appoint them and have the Senate confirm them. But the more I think about it, with consideration of the comments above, this might not be a good idea for Trump because it would be seen as a blatant attempt at a power grab.