Purifying pension

Assalaamu 'Alaykum.
My employer has set up a SIPP for me and has been paying into to it for a few years. The money is invested in a fund that also has haram income sources.

I’ve had a look at the purification course but a bit confused about how to sell the haram profits. My understanding is that you can only withdraw money after 55/56. Is it okay to do the calculation and donate the amount without selling what’s in my pension? However, the money will remain in the pension and accumulate even after switching to a halal fund.
Jazaakallah Khayr