A dependent had ITIN in 2015-2017. In 2018 he has received an SSN. Is it fine to submit 2017 taxes using his old ITIN, or new SSN should be used?

Use the new Social Security Number. Once a Social Security Number has been issued, the Social Security Administration notifies the IRS and that is the number they want you to use. 

Same scenario here, i used to enter ITIN, now am entering SSN and Federal rejected my online filing, saying it didnot match last year. What should i do ?

If the change was done very recently, the process of the Social Security Administration notifying the IRS may not have been completed. You can wait a week and try again, or paper-file and mail the return in to the IRS.