Partial take profits as stop losses

Partial TP's are great, but what would be even better is if you could (at least manually) TRAIL your partial take profits as a stop loss.

For example, all been there - price is reaching your TP1 with huge momentum and no signs of slowing down. Yes I want to take my profits, but if it looks like I can clearly get more from the market from my TP1, it'd be so nice to be able to drag my TP1 BELOW where price is currently, and instead of it greying out and not being possible, have it turn the original partial take profit buy/sell order into a stop loss order, so you can follow the momentum and squeeze more out of it. 

Currently, the only three options are either 1: to move the TP1 (or 2/3/4) up/down as the market nears it, meaning you risk not taking preferable profits if the market moves quickly against you, 2: opening up the modify order box and manually entering the position size you want to leave in the market and hover over the button ready to click it, which is clunky and slow as hell, or 3: move the stop for your WHOLE position up (or down) with the market, meaning you're out out out.

I'm an 80/20 kind of trader... 80% out at my TP1, 20% to run. If I could drag that TP1 order of 80% below price and trail it as a stop, I'd be a happy bunny. And I'm sure many could utilise this feature to great affect too!