The following are my suggestions:
Yishun:- (80X, 81X, 85X, 86X)
171 --> 850
Sembawang:- (98X, 99X)
167 --> 982
Woodlands:- (90X, 91X, 95X, 96X)
169 --> 952
178 --> 953
187 --> 951
858 --> 950
925 --> 958
926 --> 959
Note that 925 to 927 are renumbered to avoid confusion as Bukit
Panjang feeder services are 920 and 921.
Choa Chu Kang:- (87X, 88X, 93X)
67 --> 870
172 --> 874
175 --> 880
185 --> 875
188 --> 872
190 --> 871
300 --> 931
302 --> 932
307 --> 930
927 --> 881
989 --> 873
Bukit Panjang:- (92X, 97X)
75 --> 974
176 --> 971
177 --> 975
180 --> 972
184 --> 973
Bukit Batok:- (89X, 94X)
61 --> 890
77 --> 892
106 --> 891
173 --> 893
189 --> 894