I don't recommend. I am running aarch64 kernel on Raspberry Pi 4 now for about 4 months and there are no disadvantage but there are also no benefit. I do not remember how I did it exactly but I did not need any external tool.
Please also note that this will enable you only 64bit kernel. All applications (chromium for example) are still 32bit because there are no (or at least I do not remember any update on it) userland libraries like libc which also must be recompiled to aarch64. So you will run 64bit kernel, but otherwise everything non-kernel related will still be only 32bit. Even if you want compile your own hello world using aarch64 compiler it still won't work until you will install aarch64 libc to your system. Running aarch64 and armv7 shared libraries in one system is very complicated. I think that there are no benefit of running 64bit software on Raspbian and this is probably reason why migration is so slow. If you need aarch64 for some reason you can use other distributions which natively compile in aarch64 everything for raspberry pi.
I currently wait for Debian 11 release and then migrate from Raspbian to Debian. When debian become support Raspberry Pi, it will have more software and more frequently updated software than Raspbian. It will also natively run everything compiled for aarch64.
If you think about performance, consider using ramdisk and upgrading sd card rather. CPU is fast enough even in armv7 mode.
I don't recommend. I am running aarch64 kernel on Raspberry Pi 4 now for about 4 months and there are no disadvantage but there are also no benefit. I do not remember how I did it exactly but I did not need any external tool.
Please also note that this will enable you only 64bit kernel. All applications (chromium for example) are still 32bit because there are no (or at least I do not remember any update on it) userland libraries like libc which also must be recompiled to aarch64. So you will run 64bit kernel, but otherwise everything non-kernel related will still be only 32bit. Even if you want compile your own hello world using aarch64 compiler it still won't work until you will install aarch64 libc to your system. Running aarch64 and armv7 shared libraries in one system is very complicated. I think that there are no benefit of running 64bit software on Raspbian and this is probably reason why migration is so slow. If you need aarch64 for some reason you can use other distributions which natively compile in aarch64 everything for raspberry pi.
I currently wait for Debian 11 release and then migrate from Raspbian to Debian. When debian become support Raspberry Pi, it will have more software and more frequently updated software than Raspbian. It will also natively run everything compiled for aarch64.
If you think about performance, consider using ramdisk and upgrading sd card rather. CPU is fast enough even in armv7 mode.
misaz The beta images I linked to have 64-bit userlands. The 64-bit kernels on 32-bit you're talking about are the regular images on the download page? The official images are built to run on any version of the RPi.
If we're recommending third party images then Ubuntu Server images are probably the way to go: https://ubuntu.com/download/raspberry-pi (these require you to install your own desktop). If you want to use your RPi as a desktop and don't want to deal with (or feel comfortable with) having to install a desktop, then you probably want to look into something like Ubuntu MATE: https://ubuntu-mate.org/ports/raspberry-pi/.
Then again, you have to ask yourself: Are your intentions to make use of all the Raspberry Pi tutorials out there or do you want to tinker with your RPi to get more out of it?
