401 K Plan and form 5500

There is (or maybe was) a Form 5500EZ that used to be a lot easier to fill out than Form 5500. But, more to the point of the questions being asked,

Can I transfer some of the money in the 401 K plan to an IRA?

That depends entirely on the plan set-up. Most (non-solo) 401(k) plans do not allow for in-service rollovers into an IRA before age 59.5 or age 55. Solo 401(k) plans are usually set up with a large brokerage/investment house providing a prototype plan that the self-employed person adopts, with the brokerage/investment house acting as plan administrator and filing all the paperwork that administrators are required to file. So, the short answer is "Ask E*trade" to find out if your Solo 401(k) plan allows for in-service rolllovers.

If I do this, do I need to file form 5500 for the year that I do the transfer?

If I recall correctly from the good old days when I was filing Forms 5500EZ, the rules were that the form had be filed for any given year (due June 30 of the following year) if at any time during that year, the total assets of the plan exceeded the threshold. So, check what the current rules are. If rollovers are permitted and you can accomplish the rollover before there is any danger of the assets exceeding the threshold during the course of the year, do it in timely fashion instead of waiting till the balance creeps past the threshold.

Edit: OK, now that your questions regarding in-service rollovers have received a negative answer, remember that you are on the hook for filing Form 5500EZ starting from the year that you exceed the threshold. Also, please remember that there is a final 5500EZ to be filed regardless of whether the plan assets are above or below the threshold for the year in which the plan is closed down in full and all the plan assets have been distributed to the participants (including rolling over into other 401(k) plans or IRAs etc.) See this answer for some details and references]