No Stupid Questions

I won't delve into the reasons you shouldn't based on 'coolness', but...

There are half-ways between the pronunciation you're mentioning: the short 'a' sound can be used (common in British English), and wouldn't seem so jarring to me.

You can drop your Ts for weak Ds in things like 'water', so the tip of your tongue gently taps the roof of your mouth instead of pronouncing the typical British hard T (a lot of us don't pronounce it like that, mind you).

However, "speak like an American" doesn't necessarily mean one accent, as there are many in the US. Choose one, I suppose.

Foreign accent syndrome:

a medical condition in which patients develop speech patterns that are perceived as a foreign accent[1] that is different from their native accent, without having acquired it in the perceived accent's place of origin.

Foreign accent syndrome usually results from a stroke,[1] but can also develop from head trauma,[1] migraines[2] or developmental problems.[3] The condition might occur due to lesions in the speech production network of the brain, or may also be considered a neuropsychiatric condition.[4] The condition was first reported in 1907,[5] and between 1941 and 2009 there were 62 recorded cases.

Here's to becoming number 63!

Are you in the US? If so it makes a bit of sense if you want to "fit in" socially, but it's also fine if you just like the accents. If you don't live in the US then it might seem bizarre to people if you switch up your english accent (whatever it currently is). Bare that in mind.

But it's your life, you do what you want. Plenty of people have elocution lessons for various reasons. If you feel embarrassed for whatever reason then tell people you're taking elocution lessons. You could tell them you've chosen an American accent as you like the sounds, or it helps you project confidence or even because you feel it will open doors for you when communicating with other English speakers as it will remove bias or prejudice against your current accent.

There are lots of ways of justifying it if you feel the need. But hopefully your friends and family will just understand if you say you just like the way it sounds.

Scorched earth method. You are now John Black, you have no friends or family. You are an American with nothing but a pack of bubblegum and a desire to take names (of your brand new friends)

Like others, I won't comment on whether this is a good idea or not.....

Consider that all of the changes happened over time. (Unless you're speaking with a Queen's English accent, which is a posh fabrication created by the upper class BUT I DIGRESS)

So, for example, take the word "water". Look at the first syllable, 'wat'. Imagine it with a British accent, then with an American one. Think about just the vowel, the A. Try to say that vowel out loud in one accent, and hold it, then slide to the other one.

It will take lots of careful thought but you can do a similar "slow slip" for consonants and entire words too.

Good luck, I guess?

I used to have a southern accent because I grew up in North Carolina. Then I lived in Wisconsin for five years and developed a midwestern accent more or less.

It just happened naturally, just from being in a different environment.