Thanks - I have 1 contract (100 options). The 50 options that I would sell (if it were possible) means 1/2 a contract and that's what I'd like to "simulate" (returns-wise). –
Just to get on the same page and lingo, in the US, one standard option contract represents 100 shares not 100 contracts.
There are a number of things that you could do and they vary in complexity and outcome. The choice of which depends on the trade offs that you're willing to make. I'm going to keep it simple and just mention two.
The first choice is to buy an ATM put. This creates a Guts Strangle. Its advantage is that it locks in your current gain and allows you to participate to the upside. Between the strikes you'll break even and below the call's strike you'll start profiting again, should the stock collapse. You could buy an OTM put that costs less. That would lock in less, give back more if the stock dropped but give you more upside.
Example with made up numbers with calculations on an expiration basis
XYZ is $50 and you initially bought a 1 year $50 call for $5. XYZ rises to $65 in a short time and the call is now worth $16. Buy the same expiration $65 put for $6. You'd be throwing away $6 of your $11 gain. Between $50 and $65 you'd make $4. Below $50 you'd make a dollar for every $ XYZ stock dropped as you would above $65 for every $ that XYZ rose.
Plan B would be to roll the call up. At $65, the $50 call worth $16 would have a delta of about 90. The $65 call would trade at about $6 and would have a delta of about 50+ or almost 1/2 your initial delta of 90, achieving your goal of approximately 1/2 a position, so to speak. Delta is non linear so the $65 call won't track at exactly 50% but it's close enough. You'd book an $11 profit with a guaranteed $5 gain and be in approximately the same situation as when you started with the $50 ATM call.
The first suggestion has a better upside because of the much higher delta. Prior to expiration, the options would have salvage value but I'm going to leave that alone since it gets deeper into the weeds.