Just figured this out myself, the best answer is AFNI’s 3dresample:

3dresample \
-master $FSLDIR/data/standard/MNI152_T1_1mm.nii.gz \
-prefix someMaskOrParameter_1mm.nii.gz \
-inset someMaskOrParameter_2mm.nii.gz

From: <Yang>, Daniel Yang <[log in to unmask]>
Reply-To: "[log in to unmask]" <[log in to unmask]>
Date: Monday, July 7, 2014 at 11:47 AM
To: "[log in to unmask]" <[log in to unmask]>
Subject: [FSL] visualize 2mm ROI in 1mm space

Dear FSL users,

If given a 2mm ROI, how would you visualize it in 1mm space?

A related question is, how to preserve the voxel sizes if the solution involves some conversion? That is, the ROI dimension in terms of mm^3 by fslstats –V?

Many thanks,

Daniel