Question about backdoor roth ira

I have a rollover IRA in Vanguard since 2008 lets say having 20 k.

 

I create a new traditional IRA in 2024 in Vanguard and fund 7k .  Let s say it has 8k now after gains in 2025, i want to convert this account into Roth IRA .

What are the tax implications for the Rollover IRA which have had since 2008. Does any tax apply to the 20k in the Rollover IRA which i have had since 2008, when i do the conversion of new Traditional IRA into Roth IRA?