Confusion on 5 and 5.5e books as a DM

I am a DM that has been running a campaign for a couple of years. I run it fully on D&D Beyond. I have been using the 5e books for the DMG and Players Handbook. If I were to buy the 5.5e DMG and the Players Handbook would I be forced to use those rules automatically? I just am not sure how it works mechanically. I know some subclasses are missing in the 5.5 or changed and I don't know if I could use the 5e version if I (or a player) preferred that. Just not sure if things override the experience. 

Thanks, in advance.

There are settings to allow their use or not. The 2014 version is labeled as "Legacy", so it should be fairly easy to differentiate.

In essence, once you have both, you have the ability to use both.

You're a little vague as to what you mean by "forced to use those rules automatically" so I'll answer the two interpretations of that statement that spring to mind for me:

Will D&D Beyond change/default to displaying 5.5e information?
No more than it already does. The 5.5e Basic Rules are free and already available on the site. The character sheet already displays both 5e and 5.5e rules information on character sheets—weapons list their mastery traits, conditions list both 5e and 5.5e information, any rules that have variations between revisions display both versions. Additionally you can filter content in the character builder based on source. You can disable 5.5e core rules (the PHB, MM, and DMG) and 5.5e expanded rules (every books released after the 5.5e core rules). Likewise you can disable 5e core rules, 5e expanded rules (every non-core rulebook released up to the 5.5e core rules) and legacy content.

Maps uses the 5.5e encounter building rules, whereas the encounter builder uses the 5e version of those rules. The monster homebrew tools has a toggle to switch between 5e and 5.5e stat block styles.

So overall you can still use the tools to play a 100% 5e game, regardless of if you buy any 5.5e books or not.

Do I have to play using the 5.5e rules?
No, you can play using whatever ruleset you're comfortable with. While there's a recommended approach to combining 5.5e with 5e (most simply put, use the latest version of anything), you can mix and match however you wish. While some approaches may cause more mechanical friction, it's your game to construct however works best for you.

My personal recommendation is to use the 5.5e rules by default, unless an option or rule doesn't exist in 5.5e yet, in which case use the 5e version. Also don't be afraid to use the 5e version of something if another 5e option references it—for example using the conjure spells with a Circle of the Shepherd druid

DDB is not so comprehensive and integrated that it dictates most of the rules. It doesn't force you to only use one spell slot per turn, for example. It just doesn't track turns (on the character sheet, at least - it does in the Encounter Builder, but the two aren't integrated enough that the Encounter Builder will tell your character sheet that you can't use a second levelled spell that Round. Between that and the compatibility of 5e and 5.5e, it doesn't really limit which rules you use.

The only thing that is really important is your Class. The Classes are different in 5e Vs 5.5e, so you can't use 5.5e Subclasses with 5e Classes. That's dealt with at the beginning of character creation - make sure you select the correct Class (the 5e or the 5.5e version) at the beginning, then it'll guide you the rest of the way.

The last thing is that there are different options, like spells, conditions, magic items etc. You have Legacy (which is for 5e) and the ones that aren't labelled Legacy (for 5.5e or both). Just select the filter appropriately and it will remove the ones that don't apply.

So you're fine buying the new DMG, if you wish. If you don't like it, you can filter it out. To be honest, the only impact buying it has is that when you click on something that links to it, it'll actually take you there rather than asking you to buy it. So really, the only risk is your money (which is a decision you have to make, we can't tell you if you'll like it or not).