Changing the word ‘Estimate’ under your invoice customisation is the correct way to go 
This will update it on your existing and new ‘quotes’ and will show up as ‘quotes’ on the client control panel (when they log in).
However, under ‘Sales’ for example, this label remains the same:
But it will update on your quotes themselves:
I should also mention that the ‘Clone template’ option relates to the Custom Trading Styles, so you should only be using this option if you take advantage of the full custom trading styles tools. Hope that makes sense!