JavaScript (Node.js), 131 bytes

s=>unescape(s.replace(r=/B.(D.)?/g,(f,s)=>(-~g(f[1])*4*!!s+g((s||f)[1])).toString(16),g=c=>'IAOU'.indexOf(c)).replace(/../g,'%$&'))

Try it online! Link includes test cases. Alternative version, also 131 bytes:

s=>unescape(s.replace(r=/B.(D.)?/g,s=>(-~g(s[1])*4*!!s[3]+g(s[3]||s[1])).toString(16),g=c=>'IAOU'.indexOf(c)).replace(/../g,'%$&'))

Try it online! Link includes test cases.