Can I transfer Roth assets from one fund to another, without transferring pre-tax assets?

No. If your balance includes both pre-tax and Roth assets, transfers will include a proportionate amount from each contribution source.

For example: If you have $1,000 in a fund, of which $600 (or 60%) is Roth and $400 (or 40%) is pre-tax, any transfer out of the fund will consist of 60% Roth assets and 40% pre-tax assets.