No.
This is actually worse than that.
You can start by using the new version of Buildroot with your old default configuration file, but you will need to carefully check the resulting configuration for outdated packages and packages whose versions are not compatible with any software that you can add to the Buildroot file system. The names of some packages (for example, opencv) change over time, so you need to look into the resulting .config file to make sure that all the packages you need.
If you build the toolchain or Linux kernel in Buildroot (usually this is done, but usually not recommended), you need to make sure that the new configuration is configured to collect the old version of the kernel and compiler. They may be too old to create some packages in the newer version of Buildroot.
If you are updating the kernel while updating Buildroot, you need to transfer your old kernel configuration file to the new kernel version. Since kernel configuration options change frequently, you probably need to start with defconfig for your board, and then use make menuconfig to manually add the configurations you need.
Busybox is a little less volatile, so it is likely that your old config will work.
If your old Buildroot configuration uses postbuild or postimage scripts, you will need to view them, but I assume that they will not need any changes.
You should allocate at least a week for this work, maybe more, depending on the complexity of the configuration. Remember that if you are forced to use an earlier provider core due to corrections for a specific SoC, for example, for the Freescale kernel 2.6.33.9 for BSC9131, then the update you want to make may not be possible without six to twelve months of work to transfer provider kernel patches to a newer version of the kernel.
Greetings.