I would say before talking about up or down, we have to look at the last result of WP in Hougang which was a 64% and quite a peak for an opposition result. Chiam in PP got 68% in PP in 1991 and this is the second highest ever for an opposition.
Hence, given the high bar and that the seat was vacated for not-too-good reasons (said by TFBH), I expect WP's votes to drop. WP/opposition/non-PAP supporters, before firing me, do note that I am not wanting to agree with fat insects. A drop would be nothing negative - WP might still get a high 61% to 62% and I had a thread that said whatever scenario that pans out would be good for WP.