If your 401k was pre-tax. the JPM account should be a traditional IRA. If your 401k was an after-tax (designated Roth option) account, then the JPM account should be a Roth IRA.
In either case, you can ask JPM to do a rollover from the JPM IRA to the new 401k (assuming the new 401k plan accepts IRA rollovers). You want to do this directly, electronically, from one to the other, so the money never passes through your bank account. You may also be able to initiate the transaction by starting with the 401k provider and giving them instructions and the JPM account number. If you want to withdraw money from the IRA first, just tell JPM you want a withdrawal. They will be required to withhold 20% for income taxes, but you should expect your actual tax to be 22%-32% or higher, once you include the 10% penalty if you are under age 59-1/2. You will get a 1099-R from JPM and the withdrawal will be added to your next tax return as taxable income.
If you want to do a partial withdrawal, do the withdrawal first, then do a direct rollover of the rest of the money. You want to tell JPM to transfer 100% of the balance to the 401k. If you tell them to transfer a dollar amount, funny things can happen. (Such as, if the balance today is $10,000, and you ask to transfer $10,000, but the transfer takes 2 days and by tomorrow the account is worth $10,003 due to stock market changes, you will be left with a JPM IRA with $3 in it.)
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If the JPM account is not an IRA, then that means the 401k was transferred into a non-qualified account. You have 60 days to move the funds to a qualified account (a rollover). After 60 days, the transfer is taxable income to you (plus a 10% penalty if under age 59-1/2, unless you were 55 or older in the year you separated from service). Once you've paid the initial tax, you can withdraw money from a non-qualified account any time you want, and you will only pay income tax on any gain since the transfer occurred. Then, if the JPM account is not a qualified retirement account and it is more than 60 days, you can't roll it over into a 401k or any other kind of retirement account.
