In a leaked email seen by the BBC, Jo Turton said she will advise the chairman of the authority to rule the proposals as "out of order".
Changes proposed by the cabinet would see Ms Turton removed from her role.
Council leader Geoff Driver and Ms Turton declined to comment but a County Hall spokesman confirmed the situation.
In the email, Ms Turton said the plans would be "highly vulnerable" to a legal challenge if not backed up by a report assessing the "legal, financial, human resource and any other relevant implications and risks".
She said she would advise the council chairman to defer the proposals at a full council meeting on Thursday, until a full report had been prepared setting out the rationale for, and implications of, the changes.
It emerged last week that the authority's Conservative leader Geoff Driver, who is being investigated over alleged council financial irregularities, had banned Ms Turton and four others from briefings he attends.
Mr Driver, who became council leader after the Tories won a majority in elections in May, had previously been in charge until 2013.
A few weeks after the election victory he was arrested on suspicion of perverting the course of justice and witness intimidation, and later released pending further inquiries.
Three other men were also held as part of the police probe which began in 2013.
The investigation relates to allegations of financial irregularities at Lancashire County Council in the tendering of a Â£5m contract with One Connect Ltd, a joint venture between the council and BT to run some services.
The individuals banned from briefings could be witnesses in the ongoing investigation, the BBC understands.
In a statement, the council's lead member for human resources Alan Vincent said: "As the revised structure replaces the current chief executive role with a new position that requires formal financial qualifications, the intention would be to appoint to that new position on an interim basis as soon as possible."
Some senior figures in Mr Driver's own party have told the BBC they believe he should step aside while investigations are ongoing.
Plans to reorganise Lancashire County Council's senior management structure are "unlawful" in their current form, the authority's chief executive said.