Upgrading from 32 bit to 64 bit... not tried that before, where 32 bit software by default installs to C:\program files (x86)\ and 64 bit to C:\Program Files\ You'd have to fetch a 64 bit version from Berkeley. I'm running the 6.10.18 64 bit version on W7 presently. No urges to rush into 6.10.19 yet... there will be a next version for sure.

http://boinc.berkeley.edu/download_all.php

Just run the installer as nothing much can break.

There should be no need to wait until the last job is done before you uninstall the 32-bit version of BOINC.
It would be a good idea to suspend computation and also suspend network activity (Activity menu) first, and perhaps make a copy of your BOINC data directory just in case. At least make sure that you know where your BOINC data directory is, so that you can point your 64-bit client at it during the new installation process. And of course don't forget to resume activities when you are done.

You might be interested to compare your BOINC CPU benchmarks before and after the change (Advanced >> Run CPU benchmarks). You should find that the integer benchmark is much higher under 64-bit. You might also try to get some idea of your credits claimed and awarded per task or per hour before and after the change. (See your Results Status pages on WCG.org).

Note however that you will get no measurable real performance difference from upgading BOINC, because nearly all of your CPU time goes not to boinc.exe but to the science applications, and these are all still 32-bit - see Windows Task Manager >> Processes tab. However, you will probably claim and may receive more credits per hour, because BOINC clients make claims based on their benchmark scores, and the 64-bit ones are higher. You will just be contributing to inflation, so watch out or the Federal Reserve will get ya

It's not all bad news though. Just by running a 64-bit operating system, you have already sped up your crunching a little. Whenever the 32-bit science app asks Windows to do something, like give it more memory or write to a file, it gets done in 64-bit. And yes, it's already been suggested that WCG provide 64-bit versions of the science applications. Priorities.