Updating 2013 iMac from 32 bit to 64 bit

Would it be possible to change my lifetime MS Office for MAC (32 bit) to 64 bit without any need for a monthly subscription. Also, any other issues that I need to be aware of before updating?

You'd better ask this from Microsoft support because MS Office is their product.

FWIW I have used Word, Excel and PowerPoint documents with LibreOffice at home and MS Office at work without any issues so you might consider LibreOffice as an alternative (if you happen to use custom macros etc YMMV but I guess that limited to "advanced users").

p.s. macOS has been 64-bit for a long time, but macOS Mojave 10.14 was the last to support for 32-bit apps. This has upset many Mac users but on the other hand this "courage" paved the way to Silicon Macs with better power requirements (I guess Motorola/IBM PowerPC Macs was an early attempt to do this). Microsoft still has to carry that backwards compatibility baggage behind although it has some ARM support.

LibreOffice System Requirements on their download page refer to the current version (e.g. needs Catalina) and there is this version support table for reference.

Just to add: old LibreOffice versions support very old macOS versions and I have used them with no issues with current versions and MS Office files.

By trial and error I have the next several years old memo (is there an official system requirements somewhere?):

LibreOffice 4.0.6.2_MacOS_PPC works in OS X 10.5.8 (last official PowerPC version)

LibreOffice 4.3.7.2_MacOS_x86.dmg (and x86-64?) works in OS X 10.6.

LibreOffice 7.0.4 works in macOS 10.11 and 10.14.

LibreOffice 7.2.6 works in OS X 10.11.6 El Capitan.

LibreOffice 7.2.6.2 works in macOS 10.13-12.

LibreOffice 7.4.7 – Requires High Sierra or later.

LibreOffice 7.5.7 – Requires Mojave or later.

LibreOffice 7.6.2 – Requires Catalina or later.

https://downloadarchive.documentfoundation.org/libreoffice/old/

One guiding rule of macOS ownership is not to install junk. Clean My Mac X is junk and may overreach and remove viable parts of unintended, other applications.

From Microsoft:

Completely remove Office 2011 for Mac

Completely remove Microsoft 365 for Mac

Nice to hear that.

p.s. I do not recommend using Clean my Mac X because it might corrupt the system while "cleaning" it. The same goes for 3rd party virus apps. macOS does not need them and they produce only obscure errors.

You can only update a Late 2013 iMac as far as Catalina.

Microsoft has a policy of only supporting the last three major versions of macOS – and Catalina isn't one of them. It dropped off that list about two years ago, when Ventura came out. That might pose an obstacle to your plans to get a 64-bit version of Microsoft Office / 365.

You are welcome.

The last version of macOS that Office 2011 for Mac and its license supports is Mojave. Your iMac can only upgrade to macOS Catalina 10.15.7 and though Microsoft does provide an older download of MS Office 2019 for Mac for Catalina, you won't be able to install it without its license which Microsoft no longer offers, since they only support the current release of macOS and the two previous releases.

As Matti points out, there is the free LibreOffice 24.8.0 which is intentionally an MS Office clone. There will be a learning curve (augmented by LibreOffice documentation) as the application interfaces are somewhat different than Microsoft — though with reasonably good document compatibility. Since the minimum operating system supported by LibreOffice today is Catalina, you probably can expect a year before that minimum operating system requirement leaves Catalina behind for new updates.

An alternative to LibreOffice is the paid SoftMaker Office 2024 which is also an MS Office clone with a more faithful Office interface and native document support. This product is a paid product with support if that is important to you. There is a free trial period.