Should I sell my 5-room resale HDB and downgrade to a 4-room BTO HDB?

Currently, I'm in the dilemma of wanting to sell off my 5-room resale HDB in a good mature estate with 45 years of tenure left and downgrade to a 4-room BTO HDB (SBF).

With most of my children grown up in university and soon to leave the nest within the next 5 years, I thought about wanting to downgrade in hopes of reducing the bills, provide better amenities for my remaining little ones and just wanting a new change of pace.

However, the only problem going about is that I'm reaching my age (I'm 54 this year) into retirement and was unsure if I could still use my CPF and how much of my CPF money could I use to fund to pay off the new building?

Would appreciate any expertise on this issue especially in my case of being considered as a second-timer BTO buyer. Thank you in advance!