is it ok to use other korea characters when ㅢ comes after ㅎ ?
for example : ( ㅢ → ㅣ)

meeting: 회 의 = 회 이
hope: 희 망 = 히 망
we: 저 희 = 저 히
sacrifice: 희 생 = 히 생

it doesn't matter if i don't use "ㅢ " ??

Question

is it ok to use other korea characters when ㅢ comes after ㅎ ? 
for example :  ( ㅢ → ㅣ)

meeting:  회 의  = 회 이
hope: 희 망 = 히 망
we: 저 희 = 저 히
sacrifice: 희 생 = 히 생

it doesn't matter if i don't use "ㅢ " ??

Answer

No. you can not do that. It is not acceptable.

ㅢ is similar ㅡ  then ㅣ,  ㅢ is not ㅣ

It is better, just pronounce fast. ㅡ   then ㅣ

for example)

회의, 회-으이
희망, 흐이-망
저희, 저-흐이
희생, 흐이-생
의자, 으이-자
정의, 정-으이