MiHomeCare, which operates Village Home Care, said the new rates would come in as part of its renegotiated contract with Carmarthenshire council.
Two ex-workers said stress over cutting visits short because of too-long travel time contributed to them leaving.
The company said it was in discussions with revenue staff over the changes.
A spokesman for Parent company Mitie said: "The email was only sent to our people who are working on the Carmarthenshire contract and only applies to them.
"The ex-employees have raised some serious allegations. We will conduct a detailed investigation into these matters."
The company is being examined by HM Revenue and Customs over pay following an internal review examining pay at its Penarth branch in the Vale of Glamorgan.
This found workers were effectively not being paid the minimum wage as they were not reimbursed for time spent travelling between appointments.
An email to staff stated: "We are in discussions with HMRC to better understand how travel time should be accounted for across our business so we are both compliant and consistent.
"On conclusion of these discussions, we will be changing rates of pay to ensure that our staff are paid for travel time."
A care company criticised by ex-staff for not allowing them enough time to go from one visit to the next has said it is to start paying staff travel time.