HI, is there any way that I can use my 2018 Turbo Tax Account to run a rough estimate of 2019 taxes (I can manually adj for std ded, etc) without affecting 2018 return?

[xmasbaby0]

No.  If you mess around with your 2018 return you will change it, make a mess of it,  and be unable to transfer it to start your 2019 tax return.  

And you cannot start a new "dummy" online account right now, since online tax preparation is closed until the 2019 software is released in late November/early December.

You can try the tax caster tool:

https://turbotax.intuit.com/tax-tools/calculators/taxcaster/

[SteamTrain]

Nope...any messing with the current year's Online software will mess up what transfers to the next year....and destroy the file that ends up getting saved permanently at the end of every year when the software changes over to the next year.

BUT...the 2018 Online software " should be " locked now and (maybe) shouldn't let you into it to mess with it anyhow.   During this change-over period...that is not always certain as the programmers step thru their change steps...but DO Not be tempted to change anything even if it does let you in.....   In the coming weeks, the Online 2019 software should be up, but it may be really buggy and useless to you.

Those who use the Desktop/Download software every year do actually have a What-If worksheet that allows you to transfer the current tax year's data into a next year column, and then lets you change the specific entries you think might change, to see how that would affect the next year's taxes ....but the Online software is far more limited and can't do that.