So far there're no use for the old summon tickets.
However, the new update mentioned that we'll able to use leftover tickets from the current to summon from UST#4:
"Furthermore, you can use any leftover "Ultra Summon Assist Tickets #4" in the "Ultra Space-Time Summon #4", available from 09/12/2018 01:00 AM (CDT)!"
They might let us do the same with the older tickets, but I'm not holding my breath on that one.
