Because you don't post what cpu/mb you will buy, we can't say you will need the new RAM or not. Like if you will buy the intel MB, like the Z skylake MB, you will need the DDR4 RAM, if the old MB used the DDR3, but some of the skylate MB will use DDR3 RAM too.
If you will buy the AMD, you can reuse the RAM, but depends on what you want.