I heard others are saying that the upgrade is optional
That's true.
What does this mean to those who are still using the old version? Will this affect them in terms of functionality or perhaps the likelihood of not being able to connect to the block chain or poor connection to related services. I am a newbie and doesn't know much about upgrades. Hopefully somebody can help. Thanks!
Bitcoin Core is designed to be backward compatible, so there shouldn't be problem with connecting to other Bitcoin nodes (unless your version of Bitcoin Core is extremely old) and existing feature also will keep working. But you'll miss various new feature, optimization and bug fixes.
Thank you very much for this information! It's hard to just do things without much knowledge these days. A lot of upgrades are happening all around the corner and for person who doesn't know much, it is a very good idea to really ask for help. Thank you for this wonderful information.