Here is my question to this! I ordered Denise 8373 or something like that from eBay. Can I just easily replace my Denise with this new Denise from eBay without any difficulty? Would I literally have ECS instead of OCS? Would I literally be able now to have the ability to switch between PAL/NTSC when this version is currently disabled now? Will I be able to switch between ECS/OCS when I hold down the two mouse button if I install this new chip? Would I have increased color in workbench if I install this chip?
My A500 is revision 5 it can only do 1 MB CHIP RAM maximum, I am intending to buy the 512K RAM trap door to install it in my A500 in the future 
That's a lot of questions.

You should do the 1MB chip mem mod if you plan on using enhanced or high res screenmodes. 8371 only supports 512K, so you'll need to pull that out (or push it out from the bottom, PLCC chips are a pain-in-the-arse) and replace it with an 8372. After that there's a couple ways, including soldering a few spots on the motherboard, or doubling up the chips in their existing spots, etc. What you decide depends on motherboard version and technical proficiency. Personally I recommend the version of the mod that converts the 512K in the trapdoor slot into chip ram.
You can also add a "Megachip" adapter after that, which would use (typically) an 8375 Agnus, and give you 2MB.
http://en.wikipedia.org/wiki/MOS_Technology_Agnushttp://amiga.resource.cx/exp/megachipECS screenmodes ain't much to write home about, but me personally, I like 'em, and I like having my Amiga's as "up-to-date as possible", especially since you can buy those chips all day long for under $20.
You start doing these other upgrades, you might as well look at the Indivision ECS. For under $100 USD this will give you scandoubled output on every screen mode, as well as allow you to use things like the SuperPlus 800x600x16 colors mode. Make sure you upgrade to 1.10 firmware. Obviously you'll want an accelerator to use that screenmode.

Early Startup control ("holding down the two mouse buttons"), you've already got that. That's part of 3.1. You do have an actual 3.1 ROM, right? If not, go buy one. Just do it. It's under $20. Don't listen to these cheapskates who say not to. Then yes, you'll be able to hold down both mouse buttons and switch screenmodes.

Like I said, I like ECS, and feeling like my system is "more modern". But you might want to read what the actual differences are before you start waiving your hat in excitement about it.
http://en.wikipedia.org/wiki/Amiga_Enhanced_Chip_Set