ANSWER: Good question. You will want to first make the 2014 contribution to the SEP IRA. Subsequently, you can transfer the SEP IRA to the new solo 401(k). The reason you cannot make the year 2014 contribution directly to the new solo 401(k) is because the Solo 401(k) was not open until this year- 2015. Unlike a solo 401k where it has to be adopted by 12/31 in order to make contributions up until the business tax return due date plus any timely filed extensions, a SEP IRA can be opened and funded for the prior year as long as it is opened by the tax return due date plus any timely filed extensions. See IRS Publication 560 for more information on this.
SOLO 401k FAQs
Solo 401k Contribution Rules