Perhaps the rep was suggesting treating this as a bookkeeping error and recording it so that it appears on Form(s) 5498 as partially a SEP contribution and partially a regular traditional IRA contribution. Otherwise, a SEP contribution is not permitted to be recharacterized as a traditional IRA contribution. The excess SEP contribution would have to be removed as an excess contribution and then a separate regular traditional IRA contribution made.
Note that the tracking of the year for which SEP contributions are made is the responsibility of the employer (yourself as self-employed), not the custodian. If some portion of the SEP contribution in question was made in 2018, that portion can be treated as a SEP contribution for 2018 instead of for 2017. (Of course you'll need sufficient self-employment income in 2018 to support that portion as SEP contribution for 2018.)
