exchanging H2O in buffer for D2O - pH vs pD?
The articles by Glasoe,[1] Covington et. al.,[2] and (much more recently) Krężel and Bal[3] suggest in their intros that pD values can be obtained from a pH calibrated electrode by adding a correction constant of about $0.4$. This was determined by comparing the measured pH when the same amount of acid or base was dissolved in $\ce{H2O}$ and $\ce{D2O}$.
References:

P. F. Glasoe, Citation Classic - use of glass electrodes to measure acidities in deuterium-oxide. Current Contents/ Physical Chemical &amp; Earth Sciences 1979, 16, 12-12. A1979HZ34800001 (pdf) Original paper: P. K. Glasoe, F. A. Long, J. Phys. Chem. 1960, 64,:188-190. Doi:10.1021/j100830a521
A. K. Covington, M. Paabo, R. A. Robinson, R. G. Bates, Anal. Chem. 1968, 40 (4), 700–706. DOI: 10.1021/ac60260a013
A. Krȩżel, W. Bal, J. Inorg. Biochem. 2004, 98 (1), 161-166. DOI: 10.1016/j.jinorgbio.2003.10.001 Available at researchgate.net.